A semy field is one strewn or powdered with many small, identical charges. In medieval usage, any number greater than six could be blazoned “semy”. (Strictly speaking, “semy” is an adjective, not a noun: it’s from the French semée, “strewn”.)
Semy charges on the field may be drawn as whole charges, placed to fit as best they can; or as an orderly array of charges, cut off by the edges of the shield. Both depictions are period, and either may be used. While semy charges are not a field treatment, in many ways they act as though they were: e.g., semy charges on a field are always blazoned immediately after the field tincture.
Charges may themselves be charged with semy charges (e.g., a bordure mullety). In those cases, the semy charges are not cut off at the edges, but are always whole.
Semy charges may only themselves be charged if the tertiary charges remain identifiable; even then, the usage is deemed a step from period practice.
The illustration is semy of roundels. Semy fields may always simply be blazoned “semy of [charges]”, but some charges have special terms when semy. “Crusilly” is semy of crosses crosslet; “semy-de-lys” is semy of fleurs-de-lys; “goutty” is semy of gouts. In like manner, “bezanty” is semy of bezants, “billety” is of billets, “mullety” is of mullets, &c.
Astra Christiana Benedict bears: Per fess azure mullety of eight points Or and purpure crusilly Or.
Gwenlliana Clutterbooke bears: Gules semy of open books Or.
Marie de Lyon bears: Or semy of suns azure.