It's mostly a matter of degree and workmanship.
A "Bubba" job is usually a very minimal job of cutting off the longer military wood, removing the bayonet lock, and possibly bending the bolt handle on a straight bolt handle rifle.
Workmanship is often pretty rough.
A true "sporter" job is more involved, usually including a totally new sporting stock, new sights, a new blue job, and often a re-barrel to another caliber.
Workmanship here can also be rough, depending on the skills of the person doing it.
At the top end is where everything is thrown away except the action and a total custom rifle is built using the action.
These jobs are usually better quality since real gunsmith skills are needed, and often true high-end custom rifles are built that can be worth thousands of dollars.