Hello there, JulieParenti.

For the rental units under one company, you can use the Class feature to track the income services and products provided within each unit. In QuickBooks Desktop, you'll need to turn on class tracking first. Here's how:

1. Go to the Edit menu, then select Preferences.
2. Select Accounting, then go to the Company Preferences tab.>
3. Select the Use class tracking for transactions checkbox.
4. If you want a reminder when you haven't assigned a class, select the Prompt to assign classes checkbox.
5. Select OK.

Then,  set up class categories for expenses and accounts. Once done, you can assign a class to the transactions you want to use. 

You can check this article for more details: Set up and use class tracking in QuickBooks Desktop. 

Let me know if you have other questions. Take care always.