A thermostat, or stat, pocket is a hollow tube, diameter to suit the stat to be inserted into it, which sits within the bundle of elements, usually in the middle of the screwplug, but sometimes offset due to restrictions, & sometimes even tacked to an element with a spot of weld.

This has always struck me as strange, to be sensing the solution temperature in the middle of a clump of stainless steel with sheath temperatures running well into the multiple hundreds. IMHO, smart thing to do is sense solution temperature elswhere in the vessel, somewhere that gravity & heat rising will give it a chance of being more accurate.

Having said this, for the vast majority of customers, including our favourites, Brewers, a stat pocket on the immersion heater itself suffices just fine.