Hi NotSoSmart,
Simply put, each trip is given a credit value. Broadly speaking, it’s related to the number of flying hours in the trip (with a minimum value per day at work) plus a small amount for time downroute over a certain length. The exact ways it’s calculated differs slightly between longhaul and shorthaul.
CAP is the total amount of credit each pilot needs to achieve each month. It’s usually in the 80-90 hour range and varies slightly by month. JSS (the roster generator) builds you a line of work to get you up to CAP. Ground duties, part time, leave etc all attract credit as well, and this is all factored into your month’s roster.
To sum up, credit is BA’s measure of how hard it wants you to work. Therefore, ‘credit dense’ trips tend to be more popular - a lot of flying in a shorter amount of time so that you’re at home more is the logic of most. Other airlines may use ‘days at work’ (so that the shortest, easiest trips therefore become the most popular!). Each method has pros and cons.
I hope that helps.
champ