p4 diffused on the client, for example. Compare the locally modified file with the version in stock. p4 diff2used to compare two files on a server .
From the manual:
$ p4 help diff
On the client machine, diff a client file against the corresponding
revision in the depot. The file is compared only if the file is
opened for edit or a revision is provided. See 'p4 help revisions'
for details about specifying revisions.
$ p4 help diff2
'p4 diff2' runs on the server to compare one set of depot files (the
'source') to another (the 'target'). Source and target file sets
can be specified on the 'p4 diff2' command line or through a branch
view.