+P amd +P+ are pressure ratings. +P ammo is loaded above regular SAAMI pressure specs. +P+ is even higher pressure.
For example, SAAMI max for 9MM is 35,000 PSI. +P is 38,500. +P+ is even higher, but does not have a SAAMI standardized max level. A generally accepted standard is that +P+ is loaded to no more than 15% higher pressure than +P, which would mean about 44,000 PSI in 9MM. Actual +P+ 9MM is usually loaded no higher than 42,000 PSI.
Since +P+ is loaded without an industry standard, it is usually only sold to LE agencies, and only after they have signed a release of liablilty for the ammo manufacturer.
These higher pressure level can be reached by either of the ways you mention, as well as some others (hotter primers, brass with less internal volume, tighter crimp).