What is the meaning of Separation Radius for ground units?

This ties into a system in Starcraft and Starcraft 2 (and Warcraft 3) called "stacking". When you select a group of air units and order them to move, they group up more tightly while moving, and then disperse when idle at the end. Therefore, they need two radii that describe how far apart they should be - one while moving and one while idle. In contrast, ground units don't stack; they have one radius that applies both while moving and while idle.

Radius is the main property controlling how "big" the unit is, both for movement and pathing and things like considering whether it's in melee range of another unit. Separation Radius is specific to the stacking behavior.

0.5 is probably the default value of the Separation Radius property (I think that's what the gray text indicates, but it's been ages since I played around with SC2 data); since the hydralisk is presumably not configured to use stacking movement, it doesn't matter what it's set to. If it were using stacking movement, it wouldn't make sense for the value to be equal to Radius.