The phrase "additional certifications" refers to extra qualifications or licenses that someone has acquired on top of their main or basic one. It means they have obtained further special training or expertise in a particular field. Full definition
Upon graduating, she immediately completed additional certification in prenatal and postpartum massage. (waddlenswaddle.com)
Schools which offer a 30 - hour classroom instruction and 12 - hour behind - the - wheel / observation course with additional certification. (drivingschool.marketing)
The state paid to send preschool teachers back to school for additional certification and boosted teacher pay. (jonathanpelto.com)