I use Tresiba U200 for my long acting. I thought there wasn't any difference except for the amount of insulin to the filler so you only need to inject half (or a third) as much volume to get the same amount of insulin.
Like for pets, who need low doses, the vet can simply dilute U100 insulin to make it U60 or less.
U100 simply means it has 100 insulin units per ml, so U300 has 300 insulin units per ml.
It might be somewhat different for Toujeo, which as far as I know is not only U300 but also acts in a different way.