Hi, Scarlett 6i6 rev 2 here, the following might apply.
A front input combines XLR and T(R)S plug formats into one hole.
Front XLR expects mic level. In my case hardwired through the mic preamps.
Front 1/4" TS or TRS is for line level stuff.
TRS (balanced/symmetric/3-wire) has all the shielding etc. benefits of XLR, just with a different physical plug format.
Hope this gets you sorted, cheers,
Shred