FS uses a fluid (no pun intended) system of Attribute + Skill level for action rolls.
Basically, when performing an action, pick an attribute + skill combination which will make sense for it. For certain actions, it won't be much of a choice, which is especially true for combat action (requiring the Attribute + Skill combination listed in the weapon's description on pp.59-76).
An attribute of unskilled is really bad, because on top of the -20, you also have disadvantaged (with your tens and units potentially being switched if the result would be worse for you).
Edit: Simultaneous posts
Qralloq is nearly right. Melee can be based off STR, but once again, it depends on the weapons. For example, your whip definitely uses AGL. Similar thing for ranged: most uses CRD, but a few weapons will use other attributes, you just need to read your weapons's description.