Original sentence: For example, in the transformation monoid on n elements, Tn, the group of units is the symmetric group Sn.In a monoid M, H1 is called the group of units
Edited sentence: (Beware that unit does not mean identity in this context, i.) For example, in the transformation monoid on n elements, Tn, the group of units is the symmetric group Sn.In a monoid M, H1 is traditionally called the group of units.The "unit" terminology comes from ring theory.in general there are non-identity elements in H1