Hi Brian,
Leads can have transactions. For example, You can select a Lead, create an invoice, and so long as "Wait for Sync Approval" is checked, it will not sync over into QuickBooks. Estimates, Sales Orders, etc. will also function the same way.
Wait for Sync Approval is a flag in Method that is used to mark transactions that are not ready to be sent to QuickBooks. This is generally used for users to confirm the data in the transaction before submitting it to QuickBooks. Something really important to note is that if the "Wait for Sync Approval" is not checked, both the transactions AND the Lead will go into Quickbooks, and the Lead will convert to a Customer. So if you do go this route, be very careful and make sure that checkbox is checked. There are no warnings telling you that the Lead will be converted.
You should also note that any transactions done in this manner will also show as non-posted in your QuickBooks Dashboard. This could grow large depending on the number of transactions you have.
Hope this helps,
- Adam