If you have a chair that you don't have in your inventory, but you always subrent it from the same location, you should still add this item to your Products to make it easy for when you want to add it to a project.

First, create the item as you normally would by entering in a name and description.

When you hit the 'Attributes' section for this item, create a custom attribute of 'Subrent Location'

I'd suggest removing the checkbox under 'Client Visible' so your team will be able to see it, but it will not be visible to your client.
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When it comes time to mark the Quantity Posted, I would put this as 0, so you'll always be alerted that there's a conflict/ overbooking and you need to subrent it.

Now you'll be able to quickly see where you subrent it from and be alerted that you need to subrent it when you add the chair to your projects.