They can be expensive and complicated to set up but can be very useful for both donor and organization. An example would be the value of labor to build a building. The service requires specialized skills, they are provided by individuals who possess such skills, and the service would typically need to be purchased if not donated. A chart of accounts needs to be designed with future growth in mind. Generally series of numbers are reserved for assets, liabilities, net assets, revenue, and expense.
Careful thought needs to be given to use and future expansion.
This is also true of other numbering series such as part numbers, project numbers, job numbers, etc. Also because in the typical non-profit service organization salaries are the primary expense. Allocate salaries based on estimated percentages of time spent working on each program or grant. Use of these costs can be tracked manually as well. Small amounts can be distributed based on FTEs or some other appropriate cost driver.