Definition
The instrumentation surface is every analytics call that exists in your repo, considered together: each event name, each payload shape, each call site, each conditional that guards it. Most teams have never seen their full surface written down. It is implicit in the source files.
Why this concept matters
Treating instrumentation as a surface (something with shape and edges) is the move that lets you compare two states of it. Skene indexes the surface, then on every PR diffs the new surface against the indexed one.