Confused about the getBoundingClientRect() function in Javascript.

According to the official document, if const rect = getBoundingClientRect();, then we have

rect.width = rect.right - rect.left;

rect.height = rect.bottom - rect.top;

Does it means the left right position are left inclusive and right exclusive, i.e. [left, right)? Otherwise it should be

rect.height = rect.bottom - rect.top + 1;

Look at the following 5x5 map. In [inclusive, inclusive] mode, the bounding rect all the 1s should be left=2, right=4, top=3, bottom=4, and we have

width=right-left+1=4-2+1
height=bottom-top+1=4-3+1

5x5 Grid

0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 0 1 1 1
0 0 1 1 1

A more specified question:

what does rect.left mean? Does it mean that the distance to left of client container or the ith pixel? What about rect.right?