Floating P&L is always marked at the exit side of the two-way-price. A long is valued at the bid, a short at the ask. That is why a new position shows a small loss the instant it opens: you have crossed the bid-ask-spread and would pay it again to get out.
It is provisional in every sense. It moves each tick, it is not available to withdraw, and it can be wiped out by a gap without a single trade being executed in between. It does, however, count fully toward equity for margin purposes.
Floating P&L on a pair that does not include your account-currency is also exposed to the conversion rate, so the figure on screen can move slightly even when the pair itself is flat.
Example: long 0.5 standard-lot of EUR/USD from 1.0840, bid now 1.0875. That is 35 pips at $5 per pip for half a lot, so $175 floating. Balance is untouched until you close.
Related: balance-vs-equity, realised-pnl, mid-rate, two-way-price