A phylogenetic tree represents a graphical illustration of evolutionary relationships among taxa. The assignment of categorial ranks for classification, however, is not an inherent attribute of a phylogenetic tree but is based on downstream subjective decisions. 

Classifications provide simplified phylogenies at the expense of resolution: the broader the temporal bands (i.e. the fewer the cutoffs), the fewer the ranks and the poorer the resolution. In this way, classifications introduce artificial categories (grades, sections) into continuously evolving biological systems.

The ICZN (International Commission on Zoological Nomenclature) proudly claims to ensure the freedom of scientists to classify animals according to their personal philosophy. This means that the ICZN doesn´t provide any rules for assigning categorial ranks to taxa beyond the demand for monphyly. Dissatisfied with this situation, a group of taxonomists proposed a rank-free alternative taxonomic system, the PhyloCode (De Queiroz & Cantino, 2020; Laurin, 2024). 

Personally, I favour the idea of Avise & Johns (1999) to maintain categorical ranks provided that their assignment is strictly tied to taxon age as reflected by the number of genetic differences. Other features like reproductive isolation, phenotype, niche differentiation, vocalisation etc. should not be taken into account.

Timetrees offer the opportunity for clades to be ranked according to their absolute ages. To establish age-based classifications, temporal thresholds (cutoffs) must be defined. For supraspecific ranks, this approach was proposed by Willi Hennig (1966) and has repeatedly been advocated since, e.g. by Avise & Johns (1999), Holt & Jonsson (2014), Naomi (2014), Jønsson et al. (2016), and Fjeldså et al. (2020).

Before applying appropriate temporal cutoffs, chronotaxonomists will have to decide to which major groups of organisms (e.g. traditional domains, kingdoms, phyla, classes) the same set of cutoffs shall be applied. Naomi (2014), for example, proposed an extremely broad chronotaxonomic framework that covers all animals, plants and fungi. It is certainly not advisable to define a plethora of different cutoffs for numerous subordinate taxa (e.g. traditional orders, families etc.). Personally, I recommend applying sets of rank-defining cutoffs to taxa traditionally ranked as a "class" because it is part of the name "Classification".

References

Avise JC, and Johns GC (1999), Proposal for a standardized temporal scheme of biological classification for extant species, Proc. Natl. Acad. Sci. 96, 7358-63. (pdf)

De Queiroz K, and Gauthier J (1992), Phylogenetic taxonomy, Annu. Rev. Ecol. Syst. 23, 449-480. (link)

De Queiroz K, and Cantino P (2020), International Code of Phylogenetic Nomenclature (PhyloCode), 190 pages. CRC Press. Boca Raton, FL. (free online access)

Fjeldså J, Christidis L, Ericson PGP, Stervander M, Ohlson LI, and Alström P (2020), An updated classification of passerine birds, In: The largest avian radiation (Fjeldså, J, Christidis L, and Ericson PGP, eds.), pp. 45-63. Lynx Edicions, Barcelona. (link)

Hennig W (1966) Phylogenetic systematics, University of Illinois Press, Chicago, IL. (link)

Holt BG, and Jønsson KA (2014), Reconciling hierarchical taxonomy with molecular phylogenies, Syst. Biol. 63, 1010-17. (pdf)

Jønsson KA, Fabre PH, Kennedy JD, Holt BG, Borregaard MK, Rahbek C, and Fjeldså J (2016), A supermatrix phylogeny of corvoid passerine birds (Aves: Corvides), Mol. Phylogenet. Evol. 94, 87-94. (abstract)

Laurin M (2024), The Advent of PhyloCode – The Continuing Evolution of Biological Nomenclature, 226 pages, CRC Press, Boca Raton, FL. (link)

Naomi SI (2014), Proposal of an integrated framework of biological taxonomy: a phylogenetic taxonomy, with the method of using names with standard endings in clade nomenclature, Bionomina 7, 1-44. (pdf)