The most common Junior and Senior combinations are father and son, but what about in other contexts, such as grandson and grandfather (assuming that they have the same name)?
As noted in another answer, junior and senior as used in records simply to distiguish BY AGE two people of the same name AT THAT POINT IN TIME.
I've actually run across records that had a third individual, of intermediate age, who was identified as "mediocris," to distinguish him from "senioris" and "junioris". These were in church parish records, so all the individuals so identified were part of the same community and congregation, but not necessarily closely related.
The real fun starts when one of the set dies, or another same-name individual enters the records. Then the suffixes (may) shift to accomodate the new, contemporary, situation.