What is the full meaning of ZR, ZS, ZT, Z1, ZRS, ZST, ZTR used in R-X Characteristics figure?
I've found the necessary information in a book hosted on Google. There is no need to purchase the book as it can be browsed normally. Use the link posted below (made it short to look beautiful) then you will see a graphical representation related to those zones, because according to the indications, the characteristics presented above are actually zones.
Here is the link: https://goo.gl/uX4F3z