In the type description Martin Cheek informally assessed the conservation status of "N. robcantleyi" as Critically Endangered based on the IUCN criteria, while noting that the species may already be extinct in the wild. On account of its vegetative and floral features—specifically the racemose inflorescence, pronounced petioles, flattened and expanded peristome, and basal and apical lid appendages—"N. robcantleyi" is placed in the infrageneric "Regiae", a group introduced by B. H. Danser in his 1928 monograph, "The Nepenthaceae of the Netherlands Indies". This group reaches its greatest diversity on the island of Borneo, where around 12 species are currently recognised. "Nepenthes robcantleyi" and "N. truncata" are the only "Regiae" species known from the Philippines. "Nepenthes robcantleyi" shows close affinities to "N. truncata", from which it differs in several aspects of pitcher morphology. "Nepenthes robcantleyi" retains fringed wings in its mature (largest) pitchers, while in "N. truncata" these are reduced to fringe-less ridges around 1 mm wide. The peristome is another distinguishing feature: in "N. robcantleyi" it is variably flattened and 6 to even 10 cm wide at the rear, compared to only 2 cm in "N. truncata", whose peristome is rounded and semi-circular in section. The peristome ribs of "N. robcantleyi" are described as "wing-like" in section and reach a much greater level of development than in "N. truncata": they are 1–1.5 mm high and spaced 1–2 mm apart. The far less conspicuous ribs of "N. truncata" (which are cylindric in section) lie closely packed together, with no obvious separation between each rib. In describing "N. robcantleyi", Martin Cheek considered it to be closer to "N. veitchii" of Borneo, writing: "Many of the characters that separate "N. robcantleyi" from "N. truncata" are those that have defined "N. veitchii"."