PRC2 is composed of the histone methyltransferase EZH2, EED, SUZ12, and the histone binding proteins RBBP4 and RBBP7. JARID2, AEBP2, PHF1, MTF2, and PHF19 are also present in PRC2 in substoichiometric amounts.
Does this have a more specific meaning than "very little" or "very small amounts", which I presume is what it means?
Paper (paywalled): http://genome.cshlp.org/content/26/7/896.abstract
It possibly refers to fractional stoichiometry i.e. less than a molecule of these proteins per molecule of PRC2. Obviously, you cannot have half a molecule and this statement means that in a gross sample, the concentration ratio of these proteins with respect to PRC2 is less than 1:1. At a molecular scale, it would mean that these proteins may not be always associated with PRC2.
This is also in agreement with the known fact that PRC2 core components are EZH2, EED, SUZ12 (along with RbAp46/48) and the others are accessory factors (Margueron and Reinberg, 2011).