Field help (POPCRI) 

A population can be populated using membership criteria. When updating the population, all employees that meet the rule are assigned to the population. Part of the membership criteria must be that the legislation of the employee record (GESEML) must be the same as that of the population. If you have also entered a sub-population, the system also filters the list of contracts by sub-population.

Example
Population legislation = FRA
Membership Rule = [F: ID] SEX = 2 & [F:CTR]POT='PAINTER'
The population contains the contracts of female painters who work within the FRA legislation.
It is advisable to launch a batch update of populations (perhaps every night, or once a week) so that populations are kept up-to-date. To do this, use the Employee population update function (TRTGENPOP).