Usage equals count of orders.
Which orders are included?
All new orders from each sales channel connected to 1sync.io, except Amazon FBA orders, which never count toward your plan limit. Also, after you added a store to 1sync.io, system will retrieve orders from 28 days ago, these order won't count to your usage, that's bonus for free.
When the usage will reset?
Usage is reset at the start of your billing cycle each month.