OTE tag tagname.[something] DINT

Hello all,

I've never came across an OTE with a tag name like this

tagname.[something]

The tag is a DINT. What are the scare brackets for and what's
the purpose of programming a tag that way?

using a bit within the dint as a bool

Look up Indirect addressing in the help file. The something is a tag that allows you to change what address is used. It's value will determine what bit the OTE represents.
If something = 0 then Tagname.[something] = Tagname.0
If something = 5 then Tagname.[something] = Tagname.5