To lock or unlock wheel chair..

Just a quick question because I am confused. I want to see what others were taught..
1st time I got my CNA and worked as a CNA.. it was considered a huge no no to wheel a resident somewhere and then lock their wheel chair, and walk away. I know it probably depends on the circumstances, but they said if we did this then it was a form of "restraint" (which we can't do..)
Well 2nd CNA around I have been told to lock the wheel chair because we don't want them disappearing on us, or falling out (??)..

So honestly, I am confused.. it probably just depends on the patient? Common Sense? Is there any truth to the "restraint" aspect I was taught several years ago?

Too add.. I worked with dementia residents, so if we locked the chair, obviously they may be confused as to why they can't move & not be able to figure out how to wheel themselves around. Is that why I was taught that? Maybe my 2nd teacher is referring to injured patients who couldn't wheel themselves?