The replacement moment
The strongest buying signal in this category is not a segment at all. It is an event: a recorder that reaches end of support, a site added or a lease signed, an incident that made the local news, a grant round awarded, a security lead newly in post. Stated plainly, no public register lists any of that, which is exactly why it stays underworked. It is watched for, one account at a time, across a named list.
Who signs: whoever owns the estate on the day the event happens, which is why the list has to be named in advance.