inglés al español Ciencias Metalurgia / Fundición

what is the meaning for" assay portion" according to this context.

For all purposes, sampling operations
can in general be broken up into two main
stages, primary and secondary, which generate two
main groups of errors: the primary error—extraction of
a laboratory sample S1 from the lot L; and the
secondary error—extraction of an assay portion from
sample S1 (potential subdivision of the sampling
process in more stages than two, if needed in practice,