Transferable is a keyword that was introduced in the DC Origins expansion.
"Transferable" is a keyword that represents a triggered power that functions in the in-play zone. The keyword "transferable" means, "At the start of the formation step, you may transfer this card."
Transferring an equipment is a one-shot modifier. To "transfer" an equipment, the controller of the modifier attaches it to another object. In order to transfer equipment from one character to another, it must be legal to attach the equipment to the character to which it is being transferred (which includes neither character being stunned). Equipment may not be transferred from a visible character to a hidden character or vice versa (unless that equipment has concealed - optional).