Hi @rtraba,
To find more information about the _data parameter I suggest reading through the EIP:
The issue comments are a great source of information about why an EIP has specific functionality. See the following comment:
I don’t know which ERC1155 tokens use _data.
As an aside, I created a simple ERC1155 example: Create an ERC1155
Feel free to ask questions and I can improve the example.