First is important to note that an asset inventory is required for ISO 27001 only if:
there are unacceptable risks that treatment demands such inventory
there are contracts, laws, or regulations you have to follow which demands such an inventory
there is a top management decision demanding such inventory
If none of the above-mentioned situations occurs, then there is no need to keep such inventory.
In case the inventory is required, the assets should be included considering those that can affect the information you want to protect. For example, if you want to protect R&D information, you need to identify on which servers, networks, and workstations this information flows through, is processed, or stored.