Despite the fact that you're clearly not posting in good faith, I'll try and explain it to you in simple words.
We'll take somebody who is five foot six and generally wears an XL-XXL when she buys stuff online. These are of course, dresses and the like, so aren't tights. She'd like to buy some tights when she does the shopping, but the XLs in the supermarkets have a grid that says they might fit and sometimes they do, but sometimes they don't or they're so poor quality that she's often put her nails through them or they've laddered within ten minutes.
Is her inside leg measurement;
25"
27"
29"
32"
34"
Is the rise from crotch smooth? Does she have a section overhang? Does she have a sensitive spot from where endometrial tissue was introduced into the incision 15 years ago and she therefore has extremely sensitive skin there?
Is her hip measurement
40"
42"
44"
46"
48"
50"
52"
54"
Are her legs where most of her body fat is distributed, or does she have proportionally slimmer legs?
Does she have a notable difference between the top of her hips and her lower back curve with the majority of bulk carried on her abdomen, or is it mostly down to having large breasts?
Does she have a long torso with a lot of space between hip bone and ribs or is it short?
Does she have 38/40/42/44/46/48/50/52/54/56 A/B/C/D/DD/E/EE/EEE/F/FF/G/GG/H or larger breasts or does she have 38DD and her weight/size requirement is due to lipodema?
Has she had a recent appendectomy or hysterectomy and therefore needs to be sure that a waistband will not slip down or is designed to stop at mid abdomen and put pressure on a fresh scar?
Does she have endometriosis and therefore can swell from one size to looking heavily pregnant depending upon the part of her cycle she is in?
Does she have varicose veins or scarring that she would like to be hidden by tights staying the colour of the little circle on the ordering box, or is she OK if they turn into sheer tights when worn by somebody of her particular size and build?
Does she also lift weights, swim regularly, do yoga or something else that means at least part of her size is actually muscle mass, is it mostly subcutaneous adipose tissue, soft and compressible or is it visceral adipose tissue so there is no give?
Do she actually have a condition causing fluid buildup in her body?
That's why it's necessary to be able to see somebody with a similar build modelling the significantly more expensive tights and other items. Because the female body varies so much at the same BMI, just guessing that something shown on a woman of a standard/acceptable to others size (or more likely a male with shaven, very shapely legs as tights models have tended to be) might fit and paying a tenner a pair plus postage and packaging on the basis of hope alone.