Sets the client API protocol level.
The Perforce API level.
Sets the API compatibility level desired. This is useful when writing scripts using Perforce commands that do not yet support tagged output. In these cases, upgrading to a later server that supports tagged output for the commands in question can break your script. Using this method allows you to lock your script to the output format of an older Perforce release and facilitate seamless upgrades. Note that this method must be called prior to calling Connect.
See the http://kb.perforce.com/P4dServerReference/ProtocolLevels/PerforceClientLevels for the API integer levels that correspond to each Perforce release.
P4Connection Class | P4API Namespace