Generally the religion can be broken into usool and furoo (although some scholars don’t like this classification, others have allowed it).
Usool: Things from aqeedah which must be known, such as Allah is above His Throne, the pillars of emaan and things from fiqh such as there are 5 daily prayers, Fajr salaah is 2 rak’ah and so on. In these there can be no difference of opinion.
Furoo: Issues in which there can be differing in.
The difference shouldn’t cause disunity as these are issues where a difference has occurred in the past and is usually a Sunnah action, whereas the Ummah remaining united is obligatory.
An example of this is Ibn Taymiyyah (rh) who mentions that a person can leave Sunnah of prayer for unity purposes.
And Allah Knows best.
Haraam means when the legislator has ordered the leaving of an action in a clear and certain way. It would be where there is a clear text mentioning that this action is to be avoided. It’s related to the weight of the word in the Arabic language and other factors.
Makrooh means where leaving of an action has been sought but not emphasised.
It is something which is allowed and with a correct intention it can be turned into an act of worship.
Allah knows best.
A scholar of the past said regarding the difference between customary and shar’i acts:
“You should also know that the capacity which is most specific to God’s Messenger is that of legislation, because that was God’s primary objective in sending him to mankind. Indeed God has stated in the Quran that legislation was the Prophet’s exclusive quality.
“Muhammad is only an apostle” [3: 144].
For this reason we should consider the statements and actions that ensued from God’s apostle relating to the general condition of the Muslim community to be a matter of legislation unless there is circumstantial evidence to the contrary.” (Treatise on Maqasid al-Sharia, p. 50)
Thus the Asl (default) is that everything the Prophet (saw) does is to be followed and is guidance unless there is evidence to suggest otherwise. This means that the burden of proof will be on those who claim a certain action is from the Prophet’s (saw) customary acts (jibiliyya) and not those who claim that it is part of the Shari’ah.