The particular currency which is necessary to purchase and settle any given product is determined by the listing exchange, not IBKR. If, for example, you enter into a transaction to purchase a security which is denominated in a currency that you do not hold and assuming that you have a margin account and sufficient margin excess, IBKR will create a loan for those funds. Note that this is necessary as IBKR is obligated to settle that trade with the clearinghouse solely in the designated currency of denomination. If you do not wish to have such a loan created and incur its associated interest costs, you would need to either first deposit funds into your account in the required currency form and amount or convert existing funds in your account using either our IdealPro (for amounts in excess of USD 25,000 or equivalent) or odd lot (for amounts below USD 25,000 or equivalent) venues, both available through the TWS.

Also note that once you close out a security position which is denominated in a given currency, the proceeds will remain in that currency regardless of whether it is the Base Currency you've selected for your account. Accordingly, such proceeds will be subject to exchange rate risk relative to your Base Currency until such time you either perform a currency conversion or use those proceeds for another similarly denominated product.

The Standard Portfolio Analysis of Risk (SPAN) is a methodology developed by the CME and used by many clearinghouses and exchanges around the world to calculate the Performance Bond (i.e., margin requirement) on futures and options on futures which the clearinghouse collects from the carrying FCM and the FCM, in turn, from the client.

SPAN establishes margin by determining what the potential worst-case loss a portfolio will sustain over a given time frame (typically set to one day), using a set of 16 hypothetical market scenarios which reflect changes to the underlying price of the future or option contract and, in the case of options, time decay and a change in implied volatility. 

The first step in calculating the SPAN requirement is to organize all positions which share the same ultimate underlying into grouping referred to as a Combined Commodity group. Next, SPAN calculates and aggregates, by like scenario, the risk of each position within a Combined Commodity, with that scenario generating the maximum theoretical loss being the Scan Risk. The 16 scenarios are determined based upon that Combined Commodity’s Price Scan Range (the maximum underlying price movement likely to occur for the given timeframe) and Volatility Scan Range (the maximum implied volatility change likely to occur for options).

Assume a hypothetical portfolio having one long future and a one long put on stock index ABC having an underlying price of $1,000, a multiplier of 100 and a Price Scan Range of 6%.  For this given portfolio, the Scan Risk would be $1,125 scenario 14.

The Scan Risk charge is then added to any Intra-Commodity Spread Charges (an amount that accounts for the basis risk of futures calendar spreads) and Spot Charges (A charge that covers the increased risk of positions in deliverable instruments near expiration) and is reduced by any offset from an Inter-Commodity Spread Credit (a margin credit for offsetting positions between correlated products).  This sum is then compared to the Short Option Minimum Requirement (ensures that a minimum margin is collected for portfolios containing deep-out-of-the-money options) with the greater of the two being the risk of the Combined Commodity. These calculations are performed for all Combined Commodities with the Total Margin Requirement for a portfolio equal to the sum of the risk of all Combined Commodities less any credit for risk offsets provided between the different Combined Commodities. 

The software for computing SPAN margin requirements, known as PC-SPAN is made available by the CME via its website.