Members of the German Parliament are expected to vote with their party caucus. The term is Fraktionsdisziplin or caucus discipline.
Casting their individual votes in a unified way is the result of an internal process: party conventions define the political plattform; policy positions are worked out in detail; the parliamentary members are issued their guidelines for voting.
For in the end a party’s power and influence in the Bundestag is based on its ability think and act as a unified body.
Fraktionsdisziplin, however, does not mean Fraktionszwang – caucus coercion. On certain legislation Members of the Parliament can deviate from the party line.
They have what the Germans call the Prinzip des freien Mandats – principle of independent mandate, which on matters of conscience is greater than maintaining caucus discipline.
See the Christian Democrats in the Bundestag.