The Platt (or Plot – the name varied from text to text) was a large poster sized document, hung up back stage, with all details of the order of scenes for a particular play.
Some of them contain quite a lot of the casting, enabling the actors to see which scene they were in and, if playing different Parts, the order in which they appeared. Properties used by the actors are also mentioned in this document, and in three cases an indication of where there were music cues to be played, so it seems likely that this was used as a plan for actors, stage hands and musicians to operate and perform a particular play.
One Platt from that time contained marginal property notes, and all the Platts we have from then (there are six survivors) contained details of the 'personal' properties that each character would need, so it was a document used by those in charge of properties to prepare and get ready all the ancillary elements needed to perform the play: the rings, letters and money bags etc. required by the actors in any particular scene.
Here is a copy of one of the original Platts we have from that period, The Platt of The Second parte of the Seven Deadlie Sinnes. This play may have been performed at The Theatre by Strange's Men c. 1590-1, or may have been performed by The Chamberlain's Men, Shakespeare's company, also at The Theatre, in 1598. The original spelling and crossings out are included, as is the large black hole in the middle, which we presume was used to hang the Platt on a post backstage.
The use of different forms of an actor's name after a part is interesting, and some have guessed that the use of 'Mr' indicated a share-holder, the bare name that of a jobbing actor (and Richard Burbadge was one of these), and a first name (Kitt, Ned, Nick etc.) showed that it was performed by an apprentice.