The inceldom spectrum is a sub-scale for those who measure a "1" to "3" on the decile scale. It is a scale wherein one can measure how deeply one is submerged with the condition of inceldom. The person who is most deeply submerged is the truecel, i.e. someone who is never even touched someone of the opposite sex, never mind dated. The least submerged is the nearcel, who carries many characteristics of inceldom, yet has some degrees of normalcy, i.e. might have dated a few times but seems to regularly go through lengthy dry spells. The median in between these two extremes is the incel.
It can be useful to view inceldom as a spectrum because incels may experience inceldom in varying degrees of intensity. As an analogy, for example, think of poverty. There is the underclass as well as the homeless. Although they're dealing with the similar issue of poverty, the homeless person is arguably worse off than the underclass since someone in the underclass may still have a roof over his/her head. Furthermore, the homeless person may not even be the worse off on the scale of poverty. There might be some people in poverty in famine stricken areas who may look at the homeless person with envy because whilst the homeless person isn't necessarily hungry, the person suffering from malnutrition may believe they're about to die a slow death tomorrow through a lack of food.