How are quaternions specified?

nihm20

As I understand it quaternions can either be specified with the scalar part first or last as according to

q = w + xi + yj + zk
or
q = xi +yj +zk + w  

where w is the scalar part. I was setting up my signals for my signal analyzer so I was wondering in which order RobotStudio defines quaternions for orienation, does Q1 or Q4 represent the scalar part w? 

nihm20

KennyJiang

I believe it's q = w + xi + yj + zk. (1,0,0,0) corresponds to no rotation.