A subsample may be: (a) a portion of the sample obtained by selection or division; (b) an individual unit of the lot taken as part of the sample; (c) the final unit of multistage sampling. The term 'subsample' is used either in the sense of a 'sample of a sample' or as a synonym for 'unit'. In practice, the meaning is usually apparent from the context or is defined.
PAC, 1990, 62, 1193. 'Nomenclature for sampling in analytical chemistry (Recommendations 1990)' on page 1205 (https://doi.org/10.1351/pac199062061193)