Trajan had the longest inscriptions seen on coins mainly due to the many honorific titles bestowed upon him by the senate. The denarius below has a modest length inscription for Trajan. The coin which predates Maxentius by about 200 years has some of the same titles, namely IMP and CAES. The other titles are read thusly:
Obverse - Emperor's name: Traiano (Trajan coins often have his adopted father's name before = N, NER, NERVA, or NERVAE), followed by titles gained from successful military campaigns (GER - Germany, DAC - Dacia). Later Parthico was added for his campaign against Parthia. PM - PONTIFEX MAXIMUS - Highest priest of the Roman religion. TR P - TRIBUNICIA POTESTATE - Tribune of the Roman people.
Reverse: COS V = Fifth Consulship - CONSUL - The consuls were the chief magistrates of the Roman government with two appointed each year. This is a very useful tool for dating coins of Trajan. PP = PATER PATRIAE - Father of his country Guess where GW's title came from). SPQR = Senatus Populusque Romanus (The Senate and People of Rome). OPTIMO PRINC = The greatest ruler (another special title bestowed to Trajan).
Early coins of the Empire usually do not have inscriptions in the exegue. However following successful campaigns, a title can appear there. In this case DAC CAP = Dacia in Captivity. On other coins of Trajan the name of the personification appearing on the reverse, e.g. VESTA and FORT RED (Fortuna Redux) can also be in the exergue.