The transfer window is a unique feature only available within Custom Game Worlds. It is the period during the season in which a club can transfer players from other clubs into their squad.
The owner of a Custom Game World can choose within their Advanced Rules to have either one or two transfer windows during the course of the season (however it is not compulsory for them to choose a transfer window within their Game World).
If there is a transfer window within the Custom Game World, then a club will only be able to transfer players from other clubs into their squad between the Turns that the transfer window is designated as open.
A club will still be able to make and accept transfer offers outside of the transfer window, however, the offer will remain as Accepted until the transfer window opens.
An accepted transfer offer could collapse prior to the transfer window opening if one of the clubs involved has a change in management.
It is worth noting that there is a default transfer window on Turn 1 within all Game Worlds. Therefore a transfer could be completed outside of the designated transfer window within the Custom Game World.
If there is a transfer window within your Custom Game World, then the full details of when it is designated as open and how many windows there are, will be displayed within the Game World Overview.