By convention, gains are recorded positively and losses negatively, and therefore a currency trading account is a kind of income account. (mathstat.dal.ca)
In such cases, one should have a separate income account for each type of realized gains and losses. (mathstat.dal.ca)
I call these «income account values» or funny money because it's not an amount you can take out of the account. (blog.annuity123.com)