1.5 W mono output into 4 Ω — the load-dependent rating
The SSM2211CPZ-REEL7 is a Class AB audio amplifier delivering 1.5 W into a 4 Ω load from a single supply rail. That 1.5 W figure is the max output power per channel at the stated load impedance — a 8 Ω speaker will draw less current and produce lower output power for the same supply voltage, so the 4 Ω rating is the one to use when sizing the speaker for a portable audio design.
Package and board-fit — 8-LFCSP with exposed pad
The supplier device package is 8-LFCSP (3x3), and the package / case is 8-WFDFN Exposed Pad, CSP; the two descriptions refer to the same physical footprint. The -REEL7 suffix specifies the 7-inch reel quantity — the exact ordering code for automated pick-and-place lines.
Differential inputs and shutdown — what they mean for the circuit
Differential inputs reject common-mode noise picked up on the audio input traces — useful when the amplifier sits near a switching regulator or digital bus on the same PCB. Output type is 1-Channel (Mono). For stereo designs, two devices are needed — one per channel — with independent shutdown control if desired.