"Are you saying an employer can not reimburse an employee's vehicle expenses using the Actual Expenses for the vehicle?"

Why, yes, exactly 🙂

Just because I bring a Gas fillup receipt, you have no idea where I used that gas. You can, of course, throw Money at me, but that is not An accountable Plan. You don't know if I used that gas to go chop wood this weekend, and then filled up on the way to the job site just now, and I intend to go back into the woods tonight to get that wood, and then tomorrow, on the way to the shop, I intend to fill the tank again, and bring you another receipt.

I am not responding in regards to office space, which is a different concept. And a Contractor can certainly list "office space provision" on their charge to the customer, as just another justification item for why the customer should pay them more. The same is true for the comment here: "and gives the contractor a certain dollar amount per day"

Which seems like the Contractor is asking the Client to pay a Per Diem. This does not Preclude that contractor from entering for their own business tax expense the lodging and food and other travel as qualifies for their own tax return. It's just another way of expecting Revenue.

Example:

I am a Private Investigator. I will charge you $100 per day for "costs" + my Daily rate of $1,000 + "gas". That just means you have to pay me All of this for everything I turn in. It doesn't change that my own business expense tracking will be Miles, actually incurred business expenses per the IRS, etc. It just means all of that I told you is part of my Gross Revenue.

There is No Reimbursement in that paragraph. It's all Income to me. My expenses are my business and tax perspective.

Here's another common example I get:

You hire someone to do a project, they rent a scissorlift, and they buy supplies. They turn these receipts over to you as "reimbursement" along with their fee for services. You did not Rent the scissorlift; that is not your Equipment Rental Expense. That is part of the justification for the Grand Total you will pay the person that did the work, for Services. You are not Reimbursing them for your supplies. Those were Their supplies. They are showing you how much it cost, as part of the justification for the Grand Total you will pay that person.

It's all about perspective. You pay their Grand Total. On their taxes, they expense Equipment Rental and Supplies. They have a Net from this project.

If you had rented the scissorlift for "them" doing your project, and bought those supplies for your project, you would not include them on the 1099-NEC or their Payment.