In Unified Modeling Language (UML) sequence diagrams, a 
guard condition, also called as an interaction constraint, 
is a Boolean conditional expression that guards an 
interaction operand in a combined fragment. A guard 
condition contains two expressions that designate the 
minimum and maximum number of times that a loop combined 
fragment executes.

One who plays different roles
One who is interacting with the system

A condition that determines whether a certain transition 
will occur in a state diagram when an event happens
A guard condition is evaluated only when the associated
event happens.