As a noob with three games of classic BT under my belt the SPAs on the cards in the force packs are useless. There are no rules or guidance for how to apply them to a force.
So the cards do have some rules, but they are easy to miss. The rules for using the cards are in the 'A game of armored combat', so if you skipped that rulebook you can miss them. The short of it is, the side that goes first pick a card, and you get the special pilot abilities and the skills on the card, and assign it to a friendly mech. The enemy side gets to pick a mech, and they get just the skill. So if I pick a 3/4 pilot card with jumping jack, then my opponent gets a 3/4 pilot. Next, the opponent selects a card, with an SPA cost equal or less then the first card. They get that SPA and set of skills, and the first player get to upgrade a mech to just the skill. So if I pick a 1/2 maneuvering ace, then my opponent gets a 1/2 pilot.
In this way, each side has 1 mech with upgraded special skills, and each side gets the same pilot skills. There is some light strategy with the cards which can be fun, such as choosing a very low SPA card if you think your opponent is gunning for jumping jack for example, or gunning for the best pilot skill instead of useful SPAs cause you have a mech that you want upskilled. So that each time you play, even with the same mechs, the battle will be different thanks to card selection and who gets first pick.
It also means that you should only have 1 unit with special abilities to keep track of per side, which is how most of campaign operations is set up. Its only in GM'd games where you tend to see every unit with SPAs thanks to the GM's set of campaign houserules.