Sometimes the decision to choose an external thread cryovial is based on personal preference, but other times it is advisable.
The cap of an external thread cyrovial comes down the sides of the tube and creates a ledge which allows the tube to hang securely in a cryovial rack.
When storing external thread cryovials in a freezer box it is important to be sure the wells allow enough room for the tube and cap without creating a congested space.