Short story anthologies with multiple series

[Scot]

I am a fairly new goodreads librarian, and I apologize if this question has been answered. I tried searching, but none of the keyword searches I could think of gave me the answer.

I was recently reading a series in publication order, and the book began with a main character that had never been introduced before. I found out this character had been introduced in their own novella, contained within a short story anthology that had stories from multiple series in it. This made me wonder if there was an agreed-upon way to add those short story collections.

Example 1: Legends
(https://www.goodreads.com/book/show/7...)

Contains short stories from The Dark Tower (Stephen King), The Sword of Truth (Terry Goodkind), The Tales of Alvin Maker (Orson Scott Card), Majipoor (Robert Silverberg), Earthsea (Ursula K Le Guin), The Riftwar Cycle (Raymond E Feist), Discworld (Terry Pratchett), A Song of Ice and Fire (George RR Martin), Memory, Sorrow, and Thorn (Tad Wiliams), The Dragonriders of Pern (Anne McCaffery), and The Wheel of Time (Robert Jordan). However, this anthology doesn't show on the series list for any of these except Pern.

Example 2: The Cursed Carnival
https://www.goodreads.com/book/show/5...
Contains short stories from 9 different ongoing series in the Riordan Presents line, but is not listed on any of their series pages. Also contains an original short story not in any series.