Hi Alok,
It's generally depennd upon the Employee Group level. As per Personnel structure employee's employment status with the organization is been designed on the Employee Group Level , Like Permanent, Contract, Deputanist, Temporary, Seasonal, Trainee etc. So organization likes to go with the different number range grouping for the same to immediate identigication of the employee and it's also easy to reporting purpose, different reports also. But it's totally depend upon the Orgaization to Organization.
Hope so it's clear to you.
Regards, Sankarsan