I originally made this for CG official forum, but I figured I might as well post it here.
This is a table showing what weapon ranks each class can use.
The higher the rank, the stronger the weapon. R1 is the the lowest, anybody can use R1 weapons. The higher weapon rank is 5 (on the JP version at least).
Note that it's the maximum rank a class can use, it doesn't mean that it will be able to use it right away. For example, let's say you are a new Soldier, to use Rank 2 and above Swords, you'll need to attain a certain level and sometimes a higher Class Rank
R2 has a Lv10 requirement.
R3 has a Lv20 requirement.
R4 has a Lv30 requirement.
R5 has a Lv40 requirement.
Increasing your Class Rank
, from what I've been told, is done by using skills and fightings monsters, there could be quests involved as well.
(More infos when the jobs are actually available in the US version.)