Counterwallet uses the standard approach used for exchanges. As per here,
The first currency of a currency pair is called the “base currency”, and the second currency is called the “quote currency”. The currency pair shows how much of the quote currency is needed to purchase one unit of the base currency.
In XCPBTC, XCP would be the base currency and BTC the quote currency (said differently, how much BTC for 1 XCP).
BTCXCP would be the opposite (inverse) of XCPBTC: BTCXCP = [ 1 / (XCPBTC)].