Can't you choose a class for campaign?

[Rabble]

or is this feature mainly for multiplayer?

[AngryMarine]

special missions and multiplayer ..

in campaign you play as the lib ... but you can choose all weapones(a thing i rly would love on mp too)

[Mr B.]

Main reason being (I guess) is that in the SP campaign you get visions/premonitions as the librarian.

[Rabble]

All right.
Do you have to do the main campaign to do special missions?

[Mr B.]

I'm not actually sure. Completed the game before they were brought in so I'd have access to them anyway.

[✠UK_SPAWN✠]

each chapter u finish unlcoks it for SOLO Special Mission. Online they are all open anyway.