1.Changing the leave period is a automatic task.
it takes the current server date to calculate the current leave period.
so, you can change the leave period by changing the system date of the server.
2. leave carried forward -> if an employee have some leaves remaining in a leave type that has 'leave carried forward' enabled, in the date of Carry forward,
system will add those remaining leaves to the next year, same leave type. (considering rules such as 'Maximum number of leave to carry forward', etc).
that added amount can be seen as 'leave brought forward' in that next leave period leave summery and will be added to the that year entitlement.
3. available flag -> a flag that maintains for get active leave types. system will only show the leave types that have available flag sets to true, in apply leave etc.