A MappedinMapGroup is a collection of maps, usually representing the different floors of a single building. All Maps will be in a MapGroup, but if your Venue is only a single building (like a mall) you can likely ignore MapGroups entirely.
This is the internal Mappedin ID, and can be used for the lifetime of the data you get back from getVenue, but it may not be present in future calls, even if there is another MapGroup with otherwise similar properties. An ID can never be reused if it's, say, accidentally deleted. If something more durable is required, or if the data in the Mappedin CMS is being synced from somewhere else, you should finding things using the externalId property.
Maps within this MapGroup.