I agree some of these threads of this topic offered way too much information. However, sometimes it’s a bit too cold and wet to get out of the bag, put on shoes/boots, unzip tent (if it’s that cold I’m not tarping it), crawl out, relieve yourself, crawl back in, take off shoes/boots, complain about reduced body temp, climb back into bag…
Those are the times when a mayonnaise jar or wide mouthed Nalgene Cantene, or hot water bottle, or what ever you choose comes in handy. Just expose what you need to when kneeling or however you do it, and go. Much faster and there is no need to carry it with you, just dump it out in the morning where you would have gone through the night.
It really is a bit refreshing NOT to have to get too cold or wet, and by relieving yourself, you help increase your core body temperature by not having all that fluid to keep warm as well. I have to admit the idea of the hot water bottle and using it to help keep you warm afterward seemed like a good idea to me. Just hope no leaks (no pun intended) are sprung.