Pro usually stands for "This font supports many characters". This naming generally only exists for OpenType fonts.
For Western fonts, large manufacturers usually divide fonts into multiple versions such as Std and Pro. Pro will contain more characters and OT features. Some manufacturers also have versions with more characters, but the names are various. For example, Linotype has Com, W1G, W2G, etc.
Japanese fonts are much more unified, because Japanese OpenType fonts all use Adobe's CID encoding scheme, namely Adobe-Japan1, which is divided into multiple versions due to the increase in the number of characters, Std, Pro, Pr5 , Pr6 is usually used to represent Adobe-Japan1-3 to Adobe-Japan1-6 respectively, but there are some special cases. Although Lihei is not OpenType, Pro also means that it contains a large number of characters.