When Relic logs a field-level edit, it keeps both sides of the change so the history is reviewable. The oldValue is the "before" — the content the field held prior to the action. Read together with fieldName and newValue, it lets anyone auditing the record see precisely what was overwritten: this field used to say this, and now it says that. This matters most for regulated changes, such as a status correction or a NAGPRA reclassification, where being able to prove the prior state is part of compliance. It is optional because entries that aren't field edits have no prior value to capture.
Technical section
Data model, API, and internals — visible to developers.