Hi Preeti,
LTA slab fixation varies among different companies. For example, in my company, the LTA slab is fixed at 75,000. If any employee has opted for the LTA allowance (In my company, 75K is the maximum limit he can opt for), assume he opted for 50K on an annual basis. If he is able to produce bills worth 50K, then he is not taxed. In another scenario, if he produces bills worth only 10K, the balance of 40K will be taxed and paid to him at the end of the financial year.
Furthermore, the LTA is carried forward in the next 4-year block and can be claimed twice within a block of four years.
Cheers,
Nikita