This is the flat area (= 'zone') on which the spring rests (= presses) — I don't believe it means "a rest for a flat spring", not least, because this spring slip is far from being entirely flat! The point is that the flat area leaves a certain margin of error for the positioning of the spring (e.g. it doesn't need to fit exactly into a tiny notch), while the locking ridge means the pring cant just slip out again; a similar effect might have been achieved by e.g. having a half-round groove for the spring to engage in, but in this case, the designer clearly decided this was the best option; probably somewhat relaxes manufacturing tolerances on both the (moulded) ferrite core and also the (formed) spring.