I mean that latter. Having pre-built map entities inside of another pre-built map entity. Like, say you have some chairs that you built as map entities. Now, you create a house and put those chairs in it. You then build that house (with the chairs inside) as a map entity.

If you have a map-entity inside of a map entity it'll cause your map (and A6) to act strangely, and throw the W1508 error for some rason.

Hope this'll help,
FixxeR