Modifications (or modifiers) are part of the PSYC syntax. Each line in either the routing or entity header of a PSYC packet either defines a temporary variable or modifies a persistent state variable. Modifications consist of an operator, a variable name and the parameter to the operator.

Variable families, also known as types, specify ways how to apply the '+' and '-' state operators 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.

Operators and modifications are defined in Spec:Variable while the old definition still resides at