At a high level, there’s two types - exclusive and non-exclusive.
Exclusive - the team retains the rights to the player for one year. Player can’t negotiate with other teams or sign with other teams.
Non-exclusive. The team retains rights to the pkyer for one year. Player can negotiate with other teams. Franchise tagger has right to match whatever the other team offers. If player signs with other team, the tagger gets 2 first round draft picks.
In terms of salary, guaranteed in both cases. And also in both cases, equal to the average of the 5 best salaries in the league at that position or 120% of current deal, whichever is higher.
If Bell gets exclusive tags and holds out for the season (which he won’t) he wouldn’t be able to negotiate with other teams and he would probably be fined his entire salary by the Steelers.