It's the difference between allowing others to call you "the best", and allowing that title to become
part of your official name.
As Pliny records in
his Panegyric, from early in
his reign on the Senate wanted to call
Trajan "best", but
Trajan modestly refused to accept "best" as
part of
his name.
In the course of 103, after
Trajan had celebrated
his first Dacian triumph and renovated the
Circus Maximus with the addition of seats for 5000 additional spectators,
SPQR OPTIMO PRINCIPI became the
standard rev. legend on Trajan's coins in all metals, meaning "The Senate and the
Roman People (acclaim, or dedicate this coin to) the Best Emperor". Here Optimus is
still an unofficial title:
Trajan is allowing others to call him that, but is not permitting it to become
part of
his official name. Princeps is a general description of Trajan's function, "first
man in the state" or simply "ruler, emperor", not
part of
his personal name.
In 114
Trajan conquered
Armenia, the Senate once again voted him the name "Best", and this time he accepted it as
part of
his name, along with
IMP,
CAES,
AVG, and
his victory titles
GERM and
DAC. So from here on Optimus becomes
part of Trajan's nomenclature on the
obverse of
his coins, and the
rev. legend SPQR OPTIMO PRINCIPI disappears.
