The rules may be different here in Canada, but I would think the difference is your last paid day. So for instance, you give 30 days notice, resigning on the 30th. Your last work day is the 30th, so your pay is whatever is owed to the 30th plus pay for any untaken leave days.
Or, you want your last working day to be the 30th, same as above, but your resignation date to be the 10th as you will take any owed leave days to cover the period 30th to the 10th.
The amount of pay is the same in both cases but you are an employee, and therefore the company's responsibility, longer in the 2nd scenario. Which most employers would not want.
As to whether the company can decide which path, I don't know, but I would imagine they can, But may have to pay for the privilege if they decided to force the first option as they would essentially being dismissing you instead of accepting your resignation.
EDITED: 6 Sep 2019 12:30 by INDYLS