The FB2 Standing Valves are complete units that are used to land on a top no-go shoulder of F Nipple. This no-go shoulder restricts the downward movement, but these valves are not locked into F Nipple in order to restrict the upward movement. They are used to hold a column of fluid in the tubing if the well stops producing, making swabbing operations easier.
The FB2 is generally placed in the tubing string when there is a need to retain fluid from being reabsorbed back into the wellbore formation.
A good example of this would be placing a standing valve above a submersible pump.