A seat resembles a share in an IMO and masternode. The amount of seats you purchase during an IMO determine the reward you will get based on your share (1 seat = 1%).
Each IMO will have a predefined amount of seats that can be bought when the IMO is open. The sum of the purchased seats represent a share percentage you will have in the shared managed masternode.
A seat is bound to one shared managed masternode and are not exchangeable between other masternodes, secondly, seats are bound to your user account but will be exchangeable later as mentioned in our Roadmap.