The config is saved in table 'config' with fields Name, Ivalue, Tvalue and Comments.
For the row with Name='FREQUENCY', the Comments is
'Specify the frequency (days) of inventories. (0: inventory at each login. -1: no inventory)'
IMHO you have the answer : Always = 0, Never = -1, Personnalized = any number (of days) >0
For a value of 1, that's means : any agent, without --force, could send only one inventory each 1 day. When agent call the server, the server give the date/time acceptable for new inventory (= last inventory + 1 day), and the agent send the inventory only after this date/time.