Two ways of calling documents from the Instant Database screen are discussed at this link.
But if you know the name of a document you want to call
You can create a multiple choice variable where the choices are <<document calls>>. For example, let's say you have three signature blocks, one with space for 1 signor, one for 2 signors and one for 3 signors. You could create a variable like this.
[<<Sig 1>>/<<Sig 2>>/<<Sig 3>>]
If the names of the documents are too cryptic, uou could also add groupnames to the mix and use a precedent multiple choice variable to set:
[!sigs!One Signor/Two Signors/Three Signors]
[!sigs!<<Sig 1>>/<<Sig 2>>/<<Sig 3>>]
Calling a document from the answer to a question is the traditional venue of the <*Options* block, but it's nice to know that this method exists if you see a need for it.