No, not at all. As soon as you use the terms "primary IP address" and "secondary IP addresses", you imply that they are not equal. But they are equal. It's just a list of IP addresses assigned to an interface which happens to have a certain order.

Yes, that's why I wrote it should be changed to not contain the word "alias" anymore, but simply an enumerated list.

That's true. Usually if something is not documented, the behaviour is undefined.

No. It means that "delete" and "remove" are aliases for "-alias". In reality they're simply equal. ;-)

It's not documented that way. As I wrote above.

If something is not documented, that doesn't mean that it shouldn't do anything at all. In that case a _lot_ of things wouldn't work. :-)

In the case of adding something, what should be added if nothing is specified? Should the tool invent an arbitrary IP address and add it? Now that would be nonsensical.

But when removing something without specifying which one, it makes some sense to simply remove the first existing address on that interface. It would even be OK with me to remove the last one, or an arbitrary one -- I use that shortcut mostely when I need to remove the only address from an interface (or all existing addresses), so it doesn't matter.

In fact, it might also make sense to enhance the syntax to allow the specification of a number, for example "ifconfig xyz0 delete #2" would remove the second address

However, such a feature will run into problems when the set of ip addresses is not an ordered list anymore, which might very well happen in the future. Then there will be no "first" and "last" anymore, but instead the interface will just have an unordered set of IP addresses. In fact I wish that would already be the case, so people saying "primary" and "secondary" would shut up already. :-)