Interactive applications on the IBM® i generally
involve communication with:

Display files are objects of type *FILE with attribute of DSPF on the IBM i. You use display files to communicate interactively with users at display terminals. Like database files, display files can be either externally-described or program-described.
ICF files are objects of type *FILE with
attribute of ICFF on the IBM i.
You use ICF files to communicate with
(send data to and receive data from) other application programs on remote
systems (IBM i or other operating systems).
An ICF file contains the communication formats
required for sending and receiving data between systems. You can write programs
that use ICF files which allow you to communicate with (send data to
and receive data from) other application programs on remote systems.