What is the criteria for theshXlow and threshXhigh change of values by SON?

Hi LTE Experts.
What is the criteria for theshXlow and threshXhigh change of values by SON?

It should be for Load Balancing between LTE carriers in idle mode.

Ok, any idea on what is the usual criteria for this?
Like SON checks what stats to decide on this switching?

Load for example: PRB utilisation, number of users.
PUCCH resources also, number of RRC users.
SRS resources also.

Son work in idle mode as well.
Interesting functionality. Which vendor?