How do you determine which slot units you can use?

[xDestry]

I find that i can only use some and others i can, is there a level limit to theM?

[Xbob]

Hit R/L or whatever buttons you have the menu page switch buttons set to when looking at your line shield, it'll tell you which out of the 4 slots you have available.

[Zuenza]

Also,you have to use units that are equal to or lower than the grade of your lineshield.

[Xbob]

Indeed. no S-Rank units on B-Rank armor.

[JAR]

I believe the make of them also affects the compatibility as well.

[Randomness]

No, maker has no effect on compatibility