M18 batteries fit every tool on Milwaukee's M18 platform. A new pack runs an old tool and an old pack runs a new tool — what changes between them is how much work you get per charge and how hard the pack can be pushed.
CP means Compact Pack: the smallest and lightest, and the right choice on drills, drivers and lights where balance matters more than runtime. XC means Extended Capacity, a taller pack with more cells, which is what saws, grinders and pumps generally want.
HIGH OUTPUT packs deliver more current and run cooler under load, so they hold power on tools that draw hard. FORGE is the newest line, built on tabless cells for the highest output and the fastest charging of the three.
Capacities here run from CP2.0 through XC3.0, CP3.0, XC5.0, XC6.0, FORGE XC8.0 and FORGE HD12.0, in single packs, two-packs and a ten-pack.
