The ULPs allow two devices to communicate. For example, a computer sends data to a disk to be stored for later use, or the communication that takes place in a client/server relationship.
There are many standards currently defined that have been in use for years that enable interoperation, such as SCSI or IPI. A goal of Fibre Channel is to provide a structure where legacy ULPs would continue to operate, preserving the software developed in the past. In this regard then, a map will exist in the FC-4 level for every ULP that is transportable over Fibre Channel.