Position limits are defined on regulatory websites and may change periodically.  Some contracts also have near-term limit requirements (near-term position limits are applied to the side of the market for those contracts that are in the closest expiring month issued).  Traders are responsible for monitoring their positions as well as the defined limit quantities to ensure compliance.  The following information defines how position limits are calculated;

Option position limits are determined as follows:

• Bullish market direction -- long call & short put positions are aggregated and quantified in terms of equivalent shares of stock.

• Bearish market direction -- long put & short call positions are aggregated and quantified in terms of equivalent shares of stock.

The following examples, using the 25,000 option contract limit, illustrate the operation of position limits:

• Customer A, who is long 25,000 XYZ calls, may at the same time be short 25,000 XYZ calls, since long and short positions in the same class of options (i.e., in calls only or in puts only) are on opposite sides of the market and are not aggregated
• Customer B, who is long 25,000 XYZ calls, may at the same time be long 25,000 XYZ puts. Rule 4.11 does not require the aggregation of long call and long put (or short call and short put) positions, since they are on opposite sides of the market.
• Customer C, who is long 20,000 XYZ calls, may not at the same time be short more than 5,000 XYZ puts, since the 25,000 contract limit applies to the aggregate position of long calls and short puts in options covering the same underlying security. Similarly, if Customer C is also short 20,000 XYZ calls, he may not at the same time have a long position of more than 5,000 XYZ puts, since the 25,000 contract limit applies separately to the aggregation of short call and long put positions in options covering the same underlying security.

Notifications and restrictions:

IB will send notifications to customers regarding the option position limits at the following times:

• When a client exceeds 70% of the allowed limit IB will send a notification indicating this threshold has been exceeded
• When a client exceeds 95% of the allowed limit IB will place the account in closing only. This state will be maintained until the account falls below 70% of the allowed limit. New orders placed that would increase the position will be rejected.

Notes:

Position limits are set on the long and short side of the market separately (and not netted out).
Traders can use an underlying stock position as a "hedge" if they are over the limit on the long or short side (index options are reviewed on a case by case basis for purposes of determining which securities constitute a hedge).
Position information is aggregated across related accounts and accounts under common control.

Definition of related accounts:

IB considers related accounts to be any account in which an individual may be viewed as having influence over trading decisions. This includes, but is not limited to, aggregating an advisor sub-account with the advisor's account (and accounts under common control), joint accounts with individual accounts for the joint parties and organization accounts (where an individual is listed as an officer or trader) with other accounts for that individual.

Position limit exceptions:

Regulations permit clients to exceed a position limit if the positions under common control are hedged positions as specified by the relevant exchange. In general the hedges permitted by the US regulators that are recognized in the IB system include outright stock position hedges, conversions, reverse conversions and box spreads. Currently collar and reverse collar strategies are not supported hedges in the IB system. For more detail about the permissible hedge exemptions refer to the rules of the self regulatory organization for the relevant product.

OCC posts position limits defined by the option exchanges.   They can be found here.
http://www.optionsclearing.com/webapps/position-limits

Rule 407 of the New York Stock Exchange prohibits a member organization (i.e., IB) from opening a securities or commodities account or executing any transaction for an account in which an exchange member, employee associated with another exchange member or member organization or an exchange employee is directly or indirectly interested without prior written consent of the employer.  The rule also requires IB to promptly submit to the account holder's employer duplicate account statements and confirmations.

Applicants who designate employment or affiliation with another broker are required to submit a Rule 407 letter containing the email address of their organization in order to provide notification and consent to the employer and for the purpose of transmitting statements and confirmations.  If the employment is with a financial institution and  no such Rule 407 letter is submitted, IB's Compliance Department will typically contact the applicant in order to confirm that Rule 407 does not apply.

Rule 611, which is the Trade Through Exemption of SEC Regulation NMS, is very lengthy to cover in detail.  Parties interested in reading the rule in its entirely should type "SEC Rule 611" into an internet search engine.  This is the portion of the document that is pertinent to IB traders, in a nutshell:

Typically the trades involved are a multi-component trade involving orders for a security and a related derivative, or, in the alternative, orders for related securities, that are executed at or near the same time.  The SIA (Securities Industry Association) notes that the economics of a contingent trade are based on the relationship between the prices of the security and the related derivative or security, and that the execution of one order is contingent upon the execution of the other order. 

The bottom line is that when a trade is ruled R6 the SEC has granted a trade-through exemption.  This means that these execution reports do not affect the resting orders in-between the market at the time, and the R6 execution.  For example, the real market is quoting 10.50 at 10.51, and an execution is reported at 10.90.  This execution was given an R6 exemption.  A sell limit order at 10.75 would not be executed because the 10.90 execution was given an R6 status. 

Multiple accounts maintained in the same name and taxpayer ID number are grouped for purposes of applying the maximum per client protection limits of $500,000 by SIPC and $29.5 million under Lloyd’s supplementary protection. However, if you hold accounts with IB in separate capacities (for example, an account in your name, a trust account of which you are the trustee or a beneficiary, or a joint account), then each account would be protected by SIPC and the supplementary protection up to the stated limits.