Orders with different specifications (black and white printing/color printing, hard cover/soft cover, etc.) are not able to be combined on the same order.
If you are ordering proof copies and have not yet decided on your final book specifications, you may not want to assign ISBN numbers to these proofs. The ISBN number is tied to these specifications and can not be reused. For example, if you ordered a proof and got a hard cover, then you decide to order your full run with soft covers, you would need to use another ISBN.