- Instant execution: you ask for a price, the server fills at that price or sends a requote. You can set a maximum deviation.
- Market execution: you get whatever price is available when the order arrives. No requotes, but slippage either way.
- Exchange execution: used on exchange-traded instruments, not relevant to spot FX at most brokers.
- Request execution: you get a quote first, then decide. Rare now.
On instant execution, count requotes instead, and note when they cluster. The London open and news minutes are the usual suspects.
Many traders find small two-way slippage less frustrating than frequent requotes at the moments that matter, but both are costs. The point is to know the number for your account.
Which type does your broker use, and have you measured it?