Read the Variable Modifiers specification.
Some variable families, also known as types, specify ways how to apply '+' and '-' state operations on them. If such operations show up for types, that are not known or not known to provide such operations, route them transparently to the final packet destination, then drop them, without trying to process them (in terms of decentralized persistent state). If you are the final destination, drop them. Maybe leave a non-intrusive log entry for the administrator of your software.