[dayman]

It seems that they generally have more features designed to make them easier/faster to use under duress, and fewer features designed to make them concealable. Also, generally their generally more geared towards reliability than aesthetics.

"Combat" upgrades - to me anyway - might be matte finish, beveled mag wells, trimmed mag release, night sights, and more aggressive stippling/checkering.

But, I don't think there's a standard definition.