Separate Trust Document - Not a Requirement?

[MrsMacias]

Is there any reason why a plan with individual named trustees would be exempt from having a separate trustee agreement that includes provisions covering such matters as the powers and duties of trustees, investment authority, and the kinds of investments that may be made? 

Here's some background, a client of ours is changing custodial platforms and the custodian has requested a copy of the plan's trust agreement. This is very standard in my experience. I noticed we didn't have a copy of the signed trust agreement for Cycle 3 and asked the client for a copy. The advisor reached out and indicated that they thought a plan with an individual trustee wasn't really required to have a trustee agreement. The plan uses a pre-approved document and the document provider does provide a standardized trust agreement. 

I know that they are not required to use the document providers standardized trust agreement but I've never heard of a plan being exempt from having one.