(By Hedvig Skirgård): "Is there an element expressing existential predication (A book on that topic exists./There is a book on that topic.), that is different from the equative or attributive copula? It is possible that there are various existential verbs making further distinctions, temporary versus permanent existence, or that there is a special negative existential verb. The existential marker can be used for the expression of other functions, such as aspect, as long as it is not identical to the copula of identity or attribution (as is the case with is, That book is part of a series. and That book is great). The existential element need not fulfill all formal criteria to qualify as a verb. The existence of a negative existential alone is not sufficient for a 1. Locational or positional verbs are also not enough for a 1.
The Grammaticon has a concept "existive", which seems to be meant here (called "existential verb/marker/element"), although the first example is a hyparctic clause (not an existential clause). The "copula of identity/attribution" seems to mean a copula of an equational or ascriptive clause.