Check this -
CA PPM Tuesday Tip: Resources with or without Availability
https://communities.ca.com/message/241792825#241792825
Expense Resources and Roles will always have zero availability.
It is not possible to configure an Expense resource or role with availability.
You do not use Expenses for tracking work effort or hours.
However, Equipment and Material and Labor Resources all can have zero or non-zero availability.
These types of resources can be scheduled within the investment for performing work or in the case of Equipment or Materials, can be scheduled for hours of usage.
NJ