[2] Phlyctis subagelaea was formally described by the lichenologists Santosh Joshi and Dalip Kumar Upreti in 2006, based on a specimen collected from the Periyar Tiger Reserve in the Idukki district of Kerala, India.

This specimen, deposited as the holotype at LWG (National Botanical Research Institute, Lucknow), serves as the reference for this species.

The paraphyses, which are thread-like supportive filaments interspersed among the developing spores, are branched and measure 1.5–2 μm in thickness.

[2] Phlyctis subagelaea is known only from its type locality in the tropical forests of southern India, where it grows on tree bark in humid and relatively undisturbed habitats.

However, P. agelaea typically has a thallus containing norstictic acid, smaller apothecia (0.2–1 mm), and asci that may hold 2–4 spores each, distinguishing it from P. subagelaea.