Magic affinity

Magic Affinity is affinity for specific kind of magic spells. Magic user can get spells which are not part of your affinity, but you are more likely to get spells which are part of it. Person having magic affinity is almost certainly markable with spell matching that affinity. Some spells probably cannot be cast by people without the right affinity, not even by wizards.

Hereditary Magic Affinity

Magic affinities can be hereditary, that is person get an affinity due to their ancestors having it or at least some other one.

According to Heka, all hereditary magic affinities, including being a wizard or a seer, are the products of immortal ancestry.

Possibly genes

This can mean that magic affinities are based on genes. It is unlikely to be straightforward single gene, though, based on Adrian Raven's comment of "not being straightforward". It can still be multiple genes - or something different for specific magic.

Wizard

Being a wizard or seer is apparently a form of hereditary affinity, although it may not be counted as an affinity by itself.

Known magic lineages

Both the Verres and Kitsunes are known to be from families with long magic traditions. We however haven't been told if it's connected to specific magic affinity, though Nanase may have affinity for the guardian form spell. It's the kind of lineage which royals on Other half of Main Universe have, which caused Dame Tara and Andrea commenting Nanase to be royal.

Sarah has magic affinity for her spell (possibly scrying spells or something like that), which was explicitly mentioned to be part of her bloodline.

Susan and Diane both have a magic affinity for summoning, which was one of the reasons they were speculated to be twins or at least related. This was later confirmed to be a result of shared descent from Pandora Raven via her son Adrian - Diane is Adrian's daughter and Susan the descendant of a line he started in his actual twenties.

Name-based Affinity

Another kind of affinity is name-based affinity. This kind of affinity causes you to be more likely to get spell related to your name, in fact it gets in the way of other spells that person can get. It's specifically something about being given a name when born that can impact affinity, although if you change your name for deeply personal reasons it may also activate. That means you are unlikely to deliberately trigger it for yourself, but it is possible for parents to intentionally name a child in the hope of giving them an affinity and succeed.

Note that name-based affinities are rare: your name must be really absurdly on the nose or full of puns to trigger magic's "future irony" sensors.

Examples

Both Catalina Bobcat and Kitty have cat-related name-based affinities. Both were marked by Pandora Raven with cat-themed spell as a result ... on the same day.