When we use a diamond bit ? (in what case ) and who decide that we should use diamond bit ? is it the supervisor ? or any other drilling engineer ?

Usually, using daimond Bit while drill hard formations to achieve long penetrated section with less trips & less time. Running daimond bit is decided by Drilling engineer depending on geological data and off set wells in the field, considering other factors, such as (Drilling time, driiling cost, number of trips, Down hole Geological problems, Down hole tools, Hydraulic system)

We used diamont bit in case that while drilling we rich at the formation rock, and the decision to use diamaond bit is from supervisor.

with granit rocks and the supervisor decid that

I don't agree that PDC is always for hard formation. Medium formation usually used PDC in Indonesia. It depends on formation such as lithology, the problem encountered and also rate of penetration. PDC sometimes used in shale formation to avoid the bit balling (that is one of a sample). Drilling engineer, drilling supervisor and also sometimes mud engineer were involved in deciding for PDC, 

Diamond bits is generally used to drill hard formations because of the high resistance of it's PDC and it can be used many times , the decision made by supervisor in collaboration with geologist .

while drilling encounter the hard formation, tricone bit can not drill effectively So use of diamond bit can improve drilling and save money. Drilling supervisor can decide that one.

Drill bit selection is selected by Drilling engineer while job planning

use bit diamond in high compact formation (hard) , decide use bit diamond wellsite geologist or driller

we use PDC when drill hard compacted formation , the person decide that is the drilling supervisor with arnagement of drilling office and geology  

Diamond Bit use to drill a hard formation. Usually a drilling engineer will decide what bit will be use by consulting from company man and wellsite geologist in the field

diamond bit use when hard formation is present and geologist suggest that when and where will it use and also a drilling engineer