Abilities in Final Fantasy V are split into two different categories of abilities, as shown below.
Support abilities are abilities that provide special bonuses such as HP and MP bonuses, moving faster on the Overworld, or being able to find hidden pathways. These abilities must be equipped in order to be used, except for when the character is a Mime or Freelancer, in which all Support abilities are automatically active without having to equip them.
Command abilities are abilities that must be equipped in order to use, and range from using various magic spells, special Fight commands, or special commands to escape from battles quicker. For the Mime class only, they must equip Attack and Item if they wish to use them, and are the only class able to not use them if they so choose to.