Tokens are parsed and replaced with literal text, so the end result is always a single string of text.
Because of this, you can do away with any explicit concatenation, which is especially useful if you have more than two strings.
E.G. instead of
{TXTCONCAT:variableContainingOne:"{TXTCONCAT:variableContainingZero:variableContainingFive}"}
you could just do
{TXT:variableContainingOne}{TXT:variableContainingZero}{TXT:variableContainingFive}
which will produce the exact same end result.
Of course you can mix in regular literal text as well, E.G.
1{TXT:variableContainingZero}5
All parsing is completed before the end result is passed on, so it doesn't matter how you compose the text you're looking to input, as it all ends up as part of the same string anyway.