Use 片 for flat, thin things (bread slices, leaves, pills, tablets) AND for expansive areas (a stretch of land, a sea of clouds, a wave of applause). The film/movie sense extends naturally: a roll of film is one long thin sheet.
register
Northern colloquial speech sometimes shifts 片 to piān in a few movie/sheet compounds - 片子 (film), 片儿 (a slice/sheet). For 唱片, 照片, and 相片 the standard reading stays piàn; you may hear piān in Beijing speech but piàn is the safe default.
Components
pictograph
片piàn
slice; piece
A pictograph: 片 is the right half of a vertically split log, the mirror partner of 爿 (left half). The four strokes picture wood-grain on the cut face. As Kangxi radical #91 it indexes flat-sheet chars like 版 (printing block), 牌 (placard), and 牒 (slip).
Radical
SliceKangxi #91
Mirror partner of 爿 (Kangxi 90), depicting the right half of a split log. As a radical it marks characters about thin slices, slips, or flat sheet-like objects - most notably 版 (printing block / edition) and 牌 (placard, brand). Productivity is modest.
