Reference letters for EB3 (or EB2 not NIW)

My company is going to apply LC/GC for me. I have a question about the reference letters from previous employment. HR said I will need to submit reference letters (employment verification) from previous jobs at the phase of filing 1-140. Do I need to obtain nortorized employment verification? Do the reference letter need to be well-written to verify my work scope/ability etc.?