Jason,
You will need to determine which X12 version you need to use (the trading partners should be able to tell you that).
You will also need some additional documentation. A Implementation Guide (depending on the version HIPAA or non-HIPAA where you get that changes) and a Companion Guide.
The Implementation Guide tells you what the standard for that Transaction Set is (the 834 is the Transaction Set in X12 parlance).
The Companion Guide tells you how each trading partner is deploying the standard specified in the Implementation Guide. You need both because even thoug X12 is a more strictly enforced standard than HL/7 or DICOM there is room for interpretation and many Trading Partners get the interpretation off. Plus there are segments in the Implementation Guide which are not typically covered in depth in Companion Guides do the common use of those segments.
You may have to pay for the Implementation Guide – the trading partners should provide the Companion Guides.
If the Trading partners are not providing the Companion Guides I believe they are in violation of the X12 standard and I think they can be reported to X12 or HIPAA (there is little fooling around with these standards).
email: jim.kosloskey@jim-kosloskey.com 29+ years Cloverleaf, 59 years IT - old fart.