Meaning of "direct descendant" as opposed to "indirect descendant"

I know what a descendant is. A’s children, grandchildren and greatⁿ-grandchildren are all A’s descendants.

But what does direct descendant mean? It is as if some of those people are A’s direct descendants, but others are just his indirect descendants. Is direct redundant and only used as a rhetorical device?

A direct descendent is someone who can trace their lineage by "child" relationships all the way back to the desired ancestor. A non-direct descendent has to go through a "cousin" or a "by marriage" or some other non-child relationship in order to find the desired ancestor.

The phrase direct descendant is usually used in contrast to nearest descendant, particularly with regards to inheritance.

A direct descendant is the child, grandchild, etc.

The nearest descendant may be a descendant of a subject's relative such as his brother, sister, uncle, etc.

See also lineal descendant and collateral descendant.

As OP recognises, "indirect descendant" is meaningless in the biological genealogy context. But in historical genealogy, it can be used to mean descendants whose hereditary lineage back to A includes children born out of wedlock, or "unacknowledged" by the father. There are also cases of people claiming "indirect descent" where the lineage passes across siblings, cousins, etc., but I think this is stretching it for most of us - we'd probably say "Well, you're not really a descendant, then".

Also, in more "metaphorical" contexts (including "inheritance" as it relates to computer programming), there's the possibility that Class X is primarily derived from Class B, but (through multiple inheritance) also happens to include Class C which is directly derived from A.