Collaterals - definition please

I’m loving the new advanced search but please could someone explain the RM definition of collateral lines as in “Descendants and collateral lines”
I chose my grandfather (born 1881) as the start point for a descendants list and ended up with a list that included his wife’s great great great grandparents 6 generations BEFORE him (born 1680 onwards).

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Apparently, kinda like this first Google search result:

Collateral Relatives: Who They Are & Why They Matter to Genealogy - Family Tree Magazine

“Collateral relatives are family members who are not direct ancestors or descendants, such as siblings, aunts/uncles, cousins, and in-laws.”

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But shouldn’t it be descendant collateral lines? Primary person and siblings, spouse of primary and siblings, and then descendants from that generation level? If you’re going back as well as forward, you are providing everyone in the Primary person’s tree.

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