Fungal classification is mainly based on
Fungal systematics traditionally relied on morphology of sexual reproductive structures because vegetative hyphae convergent across phyla and nutritional modes overlap all being absorptive heterotrophs. Presence of ascus containing ascospores defines Ascomycota, basidium bearing basidiospores defines Basidiomycota, zygosporangium with zygospore defines Zygomycota sensu lato. Asexual conidial morphology secondary and variable. Habitat varies widely within phyla, nutrition all absorptive osmotrophic, cell wall composition all chitinous except oomycetes excluded as stramenopiles. Therefore reproductive structures provide most stable phylogenetically informative character reflecting evolutionary innovation in meiosporangial architecture.
Ref: Campbell Biology 12th ed., Chapter 31: Fungal Classification Based on Reproductive Structures; NCERT Class 11 Biological Classification