When notes are arranged slid to left and slid to right on the Guitar tab which one (note) string is first played:B G D or D G B? in the these two bars? and why?
The left part is played D-G-B and the right part is B-G-D.
You read notes from left to right, the same way we read texts.
In this tableture, you would play D, G, B in the first bar, and B, G, D in the second bar. You always play the notes from left to right. If two notes are exactly above each other, like
E:--------- B:---0----- G:---0----- D:--------- A:--------- E:---0-----
then you play the notes at the same time.