Liquid rocket engine's hot fire testings are performed both vertically and horizontally.
What is the basis of the testing orientation - from the point of view of safety, size, complexity (or ease of operation), data collected, etc.?
NOTE: There appears to a trend wherein low thrust engines are preferred to be tested horizontally while high thrust engines are preferred to be tested vertically. Also, there are a couple of cases which is contrary to this rule-of-thumb. So, what are the key decision parameters?