Dear Prasad, it is simple: usually, on-roll employees are permanent employees, and off-roll employees are contract employees. They are engaged by the contractor who has direct control over them. This practice is followed by many small and medium industries. Sometimes, the nature of work is seasonal, and they do not have enough work throughout the year. For example, soft drinks have great demand only in summer, but in winter and rainy seasons, sales are low. In such cases, the employer hires contract workers in the peak season, and in the off-season, there is no work, hence no contracts. If all employees were on the roll, the employer would have to pay wages and other benefits even when there is no work or production. Hence, they follow this system.
In some small organizations, the employer keeps on-roll employees below 20 to be exempt from PF, and the remaining are off-roll, i.e., contract employees.
Regards, Kumar