Example: My organization has different currency requirements for different aircraft resources (tail wheel, glass cockpit, etc) or an "all aircraft" currency requirement (must have flown any aircraft in last X days).
This can be defined at the aircraft resource level. Edit any aircraft resource. Navigate to the "Solo Requirements" section. Fill in the amount of hours required and the period of time. The data that you enter will apply to the aircraft resource you are editing. You can also choose other aircraft or groupings that will count towards the same currency requirement.
If you would like to enforce this requirement in scheduling, go to the Group Options page (Admin -> System Admin -> Group Options) and check the box labeled 'Currency rules enforced on resource pages'.