The easy way to remember what can and what cannot be check in is: “You can only check in items that are tangible and inventory controlled”. Put another way (in list form) it goes like this:

Inventory Item: Yes
Subrental Item: Yes
Pool Parent: No
Pool Member: Yes
Services: No
Package Parent: No
Package Item: Yes (as long as it’s tangible and inventory controlled)
Custom Items: No (the system does not know if these are physical items, services, conceptual, etc)

Here’s a visual that may help: