مَا رَصَدَ الطُّلَّابُ إِلَّا الْقَمَرَ.
The students observed only the moon, not another object.
مَا رَصَدَ الْقَمَرَ إِلَّا الطُّلَّابُ.
Only the students observed the moon, not other observers.
The first restricts the object; the second restricts the agent. If the dispute concerns what was observed, use the first; if it concerns who conducted the observation, use the second.
The form mā…illā alone does not identify the focus of restriction. The excluded alternative and the point at issue are decisive.