Binsleepin is correct,
The 'basic' is pensionable (
) and the variable is paid depending upon how much you work or the value of the trips you choose/are assigned. Less popular trips come with higher credit and higher pay. The pensionable pay was stabilised higher about 10 years ago with a reduction in the variable pay which used to make up between 40-50% of you monthly take home, good news for those joining on BARP!
The variable pay is also taxed at a different rate to the basic pay so you get to keep more of it however the HMRC expect you to spend it all out of country for the tax privilege
.