When selecting the TC checkbox for a transfer, the expression given for the transfer will be automatically multiplied with the source/donor compartment.
If the expression is constant, the transfer will be linear.
Note that the TC checkbox can only be selected when the source of the transfer is a compartment (and not a Source/Sink block).
Before Ecolego 6
The Transfer Coefficient was a type of block up to version 5. Since Ecolego 6, the transfer coefficient has been merged with the Transfer block.