Typically milspec is short hand for the following.
4150 barrel steel
proof firing
MP testing
1/7 twist
chrome lining.
The testing will catch metallurgical flaws that could cause problems down the road, which are not seen by visual inspection.
The 1/7 twist allows you to use heavy match bullets and heavy self defense ammo.
Supposedly this grade of steel is stronger than the other common grade of steel for barrels.
Typically, "civilian spec" has always had 1/9 twist barrels, not MP tested, and sometimes not chrome lined. The collapsing stocks also have a different diameter buffer tube. Once upon a time, the only "mil spec" AR15's were Colt. Now we have lots of manufactures, like Colt, Bravo, LMT, Spikes, etc.
This is why some will call a mid-length mil spec. It meets no military specs, but it has the same quality characteristics and twist rate that a military carbine would have.