When classifying trees, which characteristics are not useful?

You'd have to distinguish some how whether they are coniferous or deciduous. Do you have a list of options?

In my bio textbook it says to create a dichotomous key, after i was done this (which included the options of coniferous and deciduous trees it asks me what characteristics of trees cannot be used to classify them using the dichotomous key.

Okay, first this you need to determine in your dichotomous key is whether the thing you are looking at is a tree or plant... If you said tree, you have to determine whether it is coniferous (Evergreen) or deciduous (Hardwood)... This is where your dich. tree branches. If you choose coniferous (i.e. Evergreen), does it have long needles (White Pine) or short needles (Eastern Hemlock). Had you chosen deciduous, you'd have to ask whether it has horizontal marks in the bark (yes or no). If you said yes, does it have a wintergreen smell? if yes it is Black Birch, if now it's Yellow Birch. If you said no, does it have Vertical Ruts, if yes, are they very deep, Kind Of Deep, shallow, or none, if you chose none, you need to determine if it has a compound leaf or simple leaf, etc...

Here's a diagram

i have this key already, but it asks me which characteristics of trees are NOT useful in determining their classification

wouldn't that mean anything not mentioned in that diagram?