Incandescent light bulb
Incandescent light bulbs come in a range of shapes and sizes. The names of the shapes may be slightly different in some regions. Many of these shapes have a designation consisting of one or more letters followed by one or more numbers, e.g. A55 or PAR38. The letters represent the shape of the bulb. The numbers represent the maximum diameter, either in 1⁄8 of an inch, or in millimeters, depending on the shape and the region. For example, 63 mm reflectors are designated R63, but in the US, they are known as R20 (2.5 in). However, in both regions, a PAR38 reflector is known as PAR38.

Q: What do the numbers identify in a bulb shape designation?
the maximum diameter