A taxonomic key couplet distinguishes between a sessile wing cell and a petiolate wing cell -- but what does that mean? Look at the crossveins, not the shape of the cell itself.
Wing venation is a critical component of many insect keys. (I was accustomed to the reduced venation of Chalcidoidea but am now in a bee lab, dealing with larger and more venous hymenopterans.)
An anteriorly sessile versus anteriorly petiolate 2nd submarginal cell (2SM or SM2) is a character that is important in some sphecidoid and vespidoid groups. While working through keys and double-checking terminology, it was hard to find online guidance on the sessile/petiolate distinction.
I finally got smart and consulted the book 'Hymenoptera of the World' (Goulet & Huber 1993).
The wing on the left shows 2SM petiolate anteriorly, because it does not touch the marginal cell (M), but is connected to it by a crossvein (like a petiole). The wing on the right is an example of 2SM sessile anteriorly, because 2SM touches M, sharing a common vein.
Image from Hymenoptera of the World.
reference: Goulet, H. & Huber, J.T. (1993) Hymenoptera of the world: An identification guide to families. Agriculture Canada, Ottawa, Canada. [page 190, mutillid key]