This pattern is used in official, legal, or academic contexts to state that something is to be considered or treated as a given fact, often as a rule or assumption. It translates roughly as 'shall be regarded as' or 'it is assumed that'. In everyday conversation it sounds stiff and unnatural, so you will mostly encounter it in contracts, regulations, test instructions, or formal announcements.

The nuance is that the speaker or writer is making a formal determination: they are declaring how something is to be understood, not describing what actually happened. For example, if a test says 'this question is considered correct', it means the official stance is that it counts as correct. It can also express a hypothetical assumption, like 'let's assume that everyone has passed' in a discussion, but again, that use is still formal.

A close cousin is こととする, which is used to declare a decision or rule (e.g., 'we will meet at 9'). ものとする focuses on how something is perceived or classified, not on an action to be taken. If you are stating a rule about behavior, こととする is more natural; if you are stating how a fact is to be interpreted, use ものとする.