Each XP marks a track for an attribute or playbook. Completing an attribute track lets you increase an action rating under that attribute by one. Completing the playbook xp track lets you pick another special ability.
Does the increased attribute dot get placed before or after the line --or more relevantly, does completing the attribute xp bar and increasing an action currently with 0 rating increase the attribute rating by 1 level as well? If so, can you max out your attribute rating with just 3 level ups on the same attribute bar while having 1 action rating in each of the attribute's action plus one initial point in the fourth action of that attribute?