the purpose of the plus sign, is to help the user see related records - so if you have an order, you will probably see the related order lines
now all of this behaviour can be programmed in, and in most cases you dont want users to open tables directly, and you should access your data via forms - in this case the plus sign is at best an irrelevance, and at worst an inconvenience, as maintaining the links actually slows access down a bit.
There is a fix to get rid of the plus signs, so if your performance starts to deteriorate you can look at that - otherwise you're best ot ignore the plus sign
you probably