I am wondering if triangles $\triangle {ABC}$ and $\triangle {DEF}$ are similar, then this must mean that $\angle {C} = \angle {F}$, or does the order in which the letters of the triangle appear in the statement of their similarity have no bearing on which angles correspond to which in similar triangles?
A worksheet I am doing seems to imply that $\angle {C} = \angle {F}$ doesn't have to be the case here, but merely that one of the other angles of the triangle is equal to $\angle {C}$.
It is a Convention/Notation Issue. The Author should indicate what that Notation is.
One Common Convention is $\triangle ABC \sim \triangle XYZ \tag{1}$ indicates $A \leftrightarrow X$ & $B \leftrightarrow Y$ $C \leftrightarrow Z$
PRO :
It is easy to Identify which sides are Proportional to which sides & which angles are equal to which angles.
CON :
It gives paradoxical cases like $\triangle ABC \not \sim \triangle YZX \tag{2}$ , even though Both (1) & (2) are talking about the Same 2 triangles !
The Other Common Convention is $T_1 = \triangle ABC \sim \triangle XYZ = T_2 \tag{3}$ indicates there is some way to match the the sides & angles , & that matching will be either given or has to be Derived later.
PRO :
(1) This allows writing $T_1 \sim T_2 \tag{4}$ , without worrying about the matching.
(2) More over , we can have Propositions like this : Given a triangle $P$ , we can make a new triangle $Q$ by joining the Mid-Points of the 3 sides which will be similar & have quarter Area. Here we do not even worry about matching the angles. Proving the Proposition will involve Deriving the matching Sides.
By Extension , we do not worry about matching the Sides in our Notations too.
(3) When we have multiple matchings , we still do not worry about it. Eq Isosceles triangles can be written like this $\triangle ABC \sim \triangle ACB \tag{5}$ & Equilateral triangles can have this $\triangle ABC \sim \triangle XYZ \sim \triangle YZX \sim \triangle ZYX \tag{6}$
Summary : Both have PRO & CON : Author has to indicate the Notation used.
OP Work-Sheet might be using the other Convention.
ADDENDUM :
There are other Conventions too.
When making the Drawing , the Author may mark the Points with Prime $'$ to indicate the Corresponding Points.
In that Notation , $T = \triangle ABC \sim \triangle A'B'C' = T' \tag{7}$
Of Course , when we have Isosceles triangle or Equilateral triangle , the Corresponding Points are not unique.
We might have something like this $T = \triangle ABC \sim \triangle A'B'C' \sim \triangle A'C'B' \sim \triangle B'C'A' = T' \tag{8}$ where we are reordering the Corresponding Sides of two Equilateral triangles.
Might be useful to mention that there is a Pictorial Convention too , where Corresponding lines are marked with tick marks like this :
There are other Notational & Pictorial Conventions too.
Basically , the Author has to indicate which Convention & Notation is being used.