If you need to track non paid leaves, there are 2 different ways in which you can track these:

1. Create a Non-PTO Leave type: If non paid leaves is an established practice/policy in your company, and there are some conditions around it - a maximum number of non-PTO days, the minimum threshold of time in the company to enjoy the non-PTO time, etc- this is the best option for you. This way employees will be able to request, notify and track their non-paid leaves, as separate from other types of leaves. Managers will be able to approve and track separately too. Follow this link to create a new leave type.
2. Create a Form: If non-paid time off is not a systematic practice, but more of a once-in-a-blue-moon event, you can still keep a record of it via a Form. You can create a form with the basic information you want to record regarding the leave - dates, approvals, etc- Follow this link to create a form.

Hope this helps. :)