Helicoil threaded inserts are sized in length increments relative to their nominal diameter. For example, you can have a 1/4-20 Helicoil whose length is 1, 1.5 or 2 times the nominal diameter.
The note in your image appears to be inconsistent. The callout of MS21209F530L seems to refer to this:
which says it is a 5/16-24 x 3D helicoil.