The vendor is required to sign only if the basic contract or task order requires the vendor to sign. Many contracting officers prefer to have signed acknowledgement of order receipt so there are no misunderstandings if performance is delayed. This can also depend on how critical timely performance is for the Government. If the contract or order requires signature but the contracting officer decides he/she doesn't want to require it anymore, the contract (or task order) can be modified to remove the requirement.