This prefix indicates the namespace an OPC UA node belongs to.
In the Ignition OPC UA server all tags from devices are in index 1 and all tags from the “Exposed Tags” feature are in index 2.
Other servers have their own rules.
If you’re unsure then including the prefix is probably a better choice for you. When you drag a tag in from the OPC browser it will always include some kind of prefix. This is the best way to find out what is required or safest to use.