MLC versions 1.3 and 1.4 allow signalling for a SoundRecording whether the line-up for that recording is complete. The flag for this is called LineUpComplete.
This flag signals that the Message Sender believes that it has a complete list of all performers – featured, contracted, or not – and that these are provided in the relevant PerformingContributor tag. The flag does not relate to non-performing contributors such as studio producers or engineers (unless these persons also contribute as performers).
Note that Version 1.5 of the RDR standard will replace this single tag with two flags, one for featured contributors and one for non-featured contributors.