isReserveUnit

Given a predicate that represents a relationship or action between entities, generate a one-sentence description explaining its meaning.  
# Instructions
Focus on describing the relationship, not the entities themselves. 
# Response Format
Begin the description with \' Indicates...\'

Predicate: isReserveUnit

Indicates that one entity functions as a backup or secondary unit that can be called upon when needed, rather than serving as a primary or active unit.