Currently assertSessionPinned / assertSessionUnpinned in the spec state:

The assertSessionPinned operation instructs the test runner to assert that the given session is pinned to a mongos server.

The assertSessionUnpinned operation instructs the test runner to assert that the given session is not pinned to a mongos server.

Similarly, targetedFailPoint's wording is currently specific to mongoses:

The targetedFailPoint operation instructs the test runner to configure a fail point on a specific mongos.

Some of the transactions tests which we are expected to run against serverless (for example, this one) use these operations, even though no mongoses exist and what is really technically required to be pinned is a connection - and therefore implicitly a (non-mongos) host which the connection is to, though maybe many drivers end up pinning the host explicitly as an implementation detail.

It seems like we may want to clarify the wording on these operations to cover the difference in expectation across topologies.