The Envelopes folder contains a structure that defines the interchange. This is used if the interchange segments (ISA, GS, GE, IEA) are required. To use this, inherit from it in your own project (create a structure that customizes it). Then, in your custom structure, expand it until you see the Transaction element. You can then drag your custom transaction structure (like the 270-A1 for example) to the Transaction element which will cause it to inherit at that point. Then the custom interchange structure is ready to use.