When you're using an online booking site and you select a ticket, this often creates a hold at the airline for that fare. If you then close your browser, the hold normally remains while the airline is waiting for the agent to actually book the ticket. Eventually this times out and the fare is released back to availability.

I hope that understanding is relatively right!

You'll normally encounter this (in a negative way) when you are booking a ticket, there is only an availability of 1 for example in the fare bucket, and your browser crashes or you have problems with agent and the booking fails. Until the hold is released, you can't attempt to book that fare again.

How long is this hold generally for? Is it airline dependant? GDS dependant or Travel Agent dependant - or a combination of all?