The option for 'Timeoff allowance accrual' can be found via:

Administration -> Company -> Settings -> Timeoff settings (global)

This setting controls how many days an employee has available at a given time in the year relative to the passage of time through that year. 

For example, Joe Bloggs is entitled to 24 days per year and the holiday year begins on 1st January.

Under normal circumstances, on 1st January Joe Bloggs will be given an allowance of 24 days which he can book there and his manager can choose which to approve or decline based on company policy and the dates of the requests, but Joe still can book his full allowance.

If you set 'Timeoff allowances accrual'  to 'Yes - daily accrual'  then on 1st February Joe will only have 2 days holidays available as he has only accrued 1/12th of his allowance - as each day passes he accrues more and more allowance so, in theory, on 31st December if he has booked no holiday only then would he see an entitlement of 24 days.

This setting is not directly used when looking at new starter time off proration - by default, if someone starts mid year their starting balance for that year will be prorated accordingly. 

When adding a new employee in the middle of a holiday year you should ensure you enter their normal annual balance and not the prorated balance as the system will automatically prorate the balance looking at their start date and the start of the holiday year.

Example:

1. Joan starts work on 11th July and is entitled to 20 days in a full year.
2. The holiday year starts on 1st April.
3. From 11th July to 1st April is 101 days.
4. 101/365 days is 27.67%
5. Joan is entitled to 72.33% of her normal allowance which leaves her entitled to 14.46 days which we will then round depending on your settings for rounding time up

If you had entered 15 in the first place as her allowance, Joan would now show as only being entitled to 10.85 days which is obviously wrong and, additionally, you would then have to change her allowance at the beginning of the next year to her normal full value.