This paper describes differential transcript usage as
Differential transcript usage (DTU) is a special case of alternative splicing in which a gene’s relative transcript abundance (RTA) profile, the proportion of total gene-level expression that arises from each of the gene’s transcript isoforms, differs between conditions (Soneson et al., 2016). [...] An example of DTU may be illustrated by the scenario where the total gene expression is the same between conditions A and B, but transcript isoform 1 is the predominantly expressed transcript in condition A while transcript isoform 2 is the predominantly expressed transcript in condition B.
My question is: is differential transcript usage essentially the same as the usage levels of particular codons? If not, what is the difference? I am unsure of what "transcript isoforms" are, so I'm not sure whether it's talking about codons here or something entirely different.