In the original game, your characters started out as blank slates. They had little in the way of skills and available equipment to use. You used the License Board to gain access to new skills and equipment, but the licenses only grant your characters the ability to use/equip whatever they grant (Excluding licenses that improve items and other character abilities.) Once you've acquired a License for something, you still need to find/purchase it before you can use it. In the case of Magick and Techniks, you need only buy them once and all characters that have the proper License to use those skills can immediately make use of them.
As a result, this can result in either completely useless characters, with gear that doesn't work as well (Which is especially problematic, because there are numerous weapon types, each with their own benefits and drawbacks) or inability to use certain skills, or an utterly homogeneous party. If you spent the hundreds of hours required to, you could completely unlock the entirety of the License Board for all characters. This easily creates the scenario where all three of your party members could do anything you need them to. Need a buff? Easy, have each party member cast a different buff, the entire party will be fully buffed in no time. Enemy weak to a particular element? Great! Have each member cast the strongest spell you have of that element. Taken damage? Eh, just have them all cast Cura.
In the International Zodiac Job System there are now 12 License Boards, each corresponding to a different sign of the zodiac, and a different job. When a new character joins the party, you can assign them one of these twelve jobs, each job has their own specific License Board, this job is permanent and cannot be changed. You can even choose to not assign any jobs to anyone, but you won't learn any new abilities by doing so. Multiple characters can have the same job, but it's not recommended, as certain Esper Licenses are required to connect with Licenses that are separated from the main board. This means each character in your party will have a certain specialization, at the same time, there's now a priority in keeping certain members alive, and you'll need to worry about party composition.