A number of subtype tags are added after some monsters' general classification. With the exception of Shapechanger and a few other specific exceptions these tags are linked to a particular monster type.
For example Fiends as classification sometimes have the subtypes Devil, Demon, and Yugoloth in parenthesis after their monster type in their stat blocks.
You can consider an imp, a quasit, a hellhound, and a mezzoloth. The imp is listed as Tiny fiend (devil). The quasit is listed as Tiny fiend (demon). The hellhound has no listed subtype and the mezzoloth has the Yugoloth subtype. There is also the unusual Gnoll Fang of Yeenoghu that is listed as Medium fiend (Gnoll).
This distinction is not afforded to every common language "subtype" of monster. Hags, for instance, have their own section in the Monster Manual but are only listed as Medium Fey or, in the case of Night Hags, Medium Fiend without any listed subtype.
The following tags are the subtypes I could find listed in monsters' stat blocks
General: Shapechanger, Titan
Celestial: None, (Titan)
Fiend: Demon, Devil, Yugoloth, (Gnoll)
Humanoid: Goblinoid, Elf, Gith, Gnoll, Gnome, Grimlock, Human, Kobold, Kua-toa, Lizardfolk, Merfolk, Orc, Quaggoth, Sahuagin, Thri-kreen, Troglodyte, Yuan-ti, Aarakockra, Dwarf, etc...
Monstrosity: None, (Titan, Yuan-ti)
With the exception of "shapechanger" that is affected by a number of spells what is the significance of these tags? Does a particular subtype having a tag (such as devils and most humanoids) have any particular meaning that is different from a variety of monster not having a tag (such as hags and beholders)?