If you use a server field with the system Plugin Framework (by placing the xdmp:set-server-field code in a main module under the Plugins directory on a host), then the server field will be populated for each App Server on the host before the first request against each App Server is run.

If you set a server field to the empty sequence, it deletes the server field and it will not appear when you run xdmp:get-server-field-names.

If you set a session or server field to a value that comes from a database node (for example, from the value of a search or the fn:doc function), then that session or server field expires after the request time limit (10 minutes by default). These fields expire because they contain references to stands in the database, and as long as those references exist the stand cannot be reclaimed (merged). To keep such a node in a session or server field, make a copy of the node before adding it to the session or server field instead of using the node directly from the database.