In BRM / RSM there are actually two types of inventory.
1. Structural - the top of your hierarchy defines your product catalog. What type of inventory you have and how it is structured. Categories contain Product Families, which in turn contain Product Lines. You can even have a capacity or quantity per Product Line. This is actually sufficient to take online bookings.
Many booking systems actually stop here. Just enough to take bookings.
But the power of BRM / RSM is that we go down to actual physical items in your inventory. This gives you enormous power to control not only your bookings but how it relates to your actual physical inventory.
2. Physical Items. When one of your customer books a bike - they are really booking a Large Mountain Bike in the above example. They don't care which of your bikes they actually get. (see above) But you do.
For that reason, you will want to create actual physical items in your inventory - to be able to track actual unique physical items in your inventory.
Each of your Items will have its own unique ID. Each Item has a history - reservations and maintenance.