Firstly: Defining Najasah
Najasah linguistically: is filth.
Najasah technically: is a substance that is deemed legally  The key here is that it is a legal as opposed to a moral, cultural, or emotional judgement of a thing as being filthy. This is partly why there is no accurate one-word translation for it in English. For example, phlegm, semen, and vomit are all considered culturally filthy, yet they do not necessarily take a ruling of being najis, nor is that natural emotion of disgust to such substances part of the process through which jurists decide on their purity or najasah. In other words, it is possible for a substance to be be qadhir (culturally filthy) but tahir (legally pure) as well as najis (legally impure) but nazif (culturally clean). filthy. It has also been described as every substance which is haram to handle, not due to its sanctity, nor its disgusting nature, nor its harm to the body and mind. It has additionally been described as a legal state which denies the one described by it the ability to pray with or in it.
Secondly: The Ruling of Removing Najasah
It is wajib to remove najasah,  It is specifically so when about to perform the prayer and other acts necessitating a state of physical purity. and this is the position of the majority: Hanafis, Shafi`is, Hanbalis, and an opinion among Malikis.
Thirdly: The Ruling of Having an Intention prior to Removing Najasah
An intention is not a condition to remove najasah, and this is by agreement of the four schools of jurisprudence.  The relied upon position among Malikis is necessitating an intention when performing istinja’ from pre-seminal fluid.
Fourthly: Doubting the Existence of Najasah
Whoever is doubtful over the purity or najasah of some substance builds upon certainty; if the substance is pure in its natural original state, then it is pure, and if it is najis in its natural original state, then it is najis. This is the position of the majority: Hanafis,  The Hanafis may put forward that which is pure with accompanying evidence. Shafi`is, and Hanbalis.
Fifthly: Benefiting from Najis Substances
It is permissible to benefit from najasah in other than clothing, food, and drink, and this is the position of the Shafi`is, an opinion among Malikis, and Imam Ahmad hinted towards it in a narration from him. Some of the Salaf had this as their verdict, and it is the choice of al-Tabari, Ibn Taymiyyah, and Ibn `Uthaymin.