Wrist pillow, wrist hanging and elbow hanging are all knowledge of calligraphy, while sitting posture and standing posture are the contents of writing posture in calligraphy. The two are related, but there are also differences.
Generally speaking, if we use the method of pillow wrist to write, we mostly use sitting posture now (there were no perfect desks and stools before the Song Dynasty, so writing in that era was basically done by hanging elbows). Just like writing in lower case now.
Hanging the elbow, as the name implies, is to hang your arm down when writing, so as not to let your whole arm touch the desktop and paper. Generally speaking, hanging elbows is for the convenience of writing, especially when writing big characters, hanging elbows helps to write freely. It is a way that many calligraphers often adopt.
Especially when writing a list of big characters, it should be said that the method of hanging elbows must be adopted to meet the needs of writing.
When writing banners or stalls, considering the visual relationship, standing will be better than sitting. Of course, if you have finished the layout and folded the paper, you can also write by sitting, but judging from the echo of gestures in the whole writing, standing writing is really better than sitting writing.
For children, you can write standing, that is, standing and practicing hanging elbows. Sometimes children are not tall enough to reach the desktop, so they have to stand and write, which virtually exercises their ability to stand and write. With the ability to write standing up, they can write freely when they sit down, whether hanging their elbows or wrists. However, this does not mean that adults should sit and write, and children should stand and write. This is just a way under special circumstances.
We often see some adults standing and writing, because they often write long cursive scripts. When writing this cursive script, sitting and writing, whether hanging your elbow or wrist, can't meet the needs of writing. And some children can sit and write, which doesn't mean that children must stand and write.
So, I don't think there is a strict boundary between standing and sitting. You can take it if you like. Generally speaking, cursive script is suitable for standing and regular script is suitable for sitting. I always think that the writing posture of standing and sitting should be chosen according to the written font. ?