Ability scores define the raw potential a character has to do anything in the campaign.

Accuracy ~ Defines the character's ability to put down accurate ranged attacks.
On battletech missions it defines their ability to attack with weapons.
On mechwarrior missions it defines their ability to attack with personal weapons.

Physical ~ Defines the character physical prowess.  Their ability to attack in close quarters, their ability to move, their ability to absorb physical punishment.
On battletech missions it defines their ability to gain initiative, make physical attacks, make pilot rolls, and absorb mechwarrior damage.
On mechwarrior missions, it also determines the number of action points the character.

Mental ~ Defines the character's mental ability.  Their general mental competence which includes things like the ability to use lead, use technical equipment, or repair their mech.

On battletech missions they are used for Leadership, Scout checks, and Electronic warfare.  Off the battlefield, they increase the modifications allowed to a mech, availability checks for equipment, repairs for mechs, and battlefield salvage rolls.

On mechwarrior they cover all mental challenges, including perception, persuasion, technical challenges, and so forth.

00 xp to have a score of 0, total cost  00 xp, skill base tn 8+

25 xp to have a score of 1, total cost  25 xp, skill base tn 7+

25 xp to have a score of 2, total cost  50 xp, skill base tn 6+

50 xp to have a score of 3, total cost 100 xp, skill base tn 5+

100xp to have a score of 4, total cost 200 xp, skill base tn 4+

150xp to have a score of 5, total cost 350 xp, skill base tn 3+

200xp to have a score of 6, total cost 550 xp, skill base tn 2+

The base skill tn, presumes that the character has been trained with the skill in question.  If they have not, the game master may allow a base roll with an +2 penalty for being untrained for basic success.

(ie.  A mechwarrior might make an untrained mental skill check to provide the technical skill needed to weld some armor onto their damaged mech after a fight.  They're not going to be able to work out what needs to be done when their mechs center torso is gutted.  The steps to rebuild the internals... attachs the limbs... install a new engine and gyro... and so forth will new a trained technician to work out the repair details.)