Two equally eligible workers want the same holiday period. An organization could leave the choice to a manager, rotate turns, or use seniority. Under a seniority rule, a specified measure of service establishes priority before the particular dispute arises.
Formal service rules can define seniority through dates of appointment or promotion, with additional rules for ties. Different institutions calculate it differently; the word alone does not identify the ordering.
Imagine a workplace using length of service for choosing shifts. Experienced workers can anticipate the benefit of remaining, and managers have less room to reward favorites in each individual decision. Newer workers, however, may repeatedly receive the least convenient options even when their circumstances are difficult.
Seniority is therefore a choice about what should count in a particular allocation. It can recognize continuity and protect against arbitrary discretion, but time served does not necessarily establish present skill or readiness for greater responsibility.
Merit-based selection instead makes relevant capability central to a choice. A seniority rule is clearest when its purpose is bounded. Using seniority to order one set of preferences is different from assuming the longest-serving person should lead every project. Predictability is its distinctive contribution; broader judgments about capability and need may require other mechanisms.