Pitbull 4 Luatext

[ComputerNerd]

Having made an effort to try and utilise Luatext to customize my unit frames, I am finding it rather slow going with a lot of things simply not working or throwing out errors.
Most notably I seem unable to concatenate two variables.
I can easily concatenate two fixed or static strings with something like "a".."b", but instead get errors doing that with variables.
Anyone got any ideas and maybe point to some good reference material ?

Edit: Found a way round it.
Was intending to shorten the classification to "+" and "R" for elite and rare, but for some reason could not cocatenate those strings together when I put them into variables, so instead I took the classification string and replaced the instances of "Elite", "-Elite" and "Rare" with the "+" and "R" respectively using string.gsub.

Still if anyone has any idea why that would not work I would appreciate it since I am likely to need that working elsewhere.