if "always headers response table" includes 2xx, how it can not be a super set of "onsuccess"?

apache official documentaion says that:

• "onsuccess (default)" responses headers table → just for 2xx responses
• "always" responses headers table → for all responses including 2xx

later it says:

always is not a superset of onsuccess

please consider that i've red full document so please do not repeat quotes of apache documents, i need answer not quotings :), thanks for your time

Solution

You missed an import part of the statement

always is not a superset of onsuccess with respect to existing headers:

It is a little confusing, but if you read the documentation you linked to it does explain it.

This difference between onsuccess and always is a feature that resulted as a consequence of how httpd internally stores headers for a HTTP response, since it does not offer any "normalized" single list of headers.

example

Here's my attempt to make the example from the docs more clear:

step 1 - Your Application

   response.status = 200
   response.headers['X-Foo'] = 'bar'
   return response

step 2: mod_proxy
mod_proxy sets headers in the always table and not in the default onSuccess table

apache header tables:

Always	onSuccess
`x-Foo: bar`

step 3: mod_headers

Header onSuccess set X-Foo: baz

apache header tables:

Always	onSuccess
`x-Foo: bar`	`X-Foo: baz`

step 4: server response

HTTP/1.1 200 
 Server: apache
 x-Foo: bar
 X-Foo: baz

summary

That's the reason the doc state

always is not a superset of onsuccess with respect to existing headers.

and that

repeating this directive with both conditions makes sense in some scenarios

Again from the example, you can resolve the issue with:
mod_headers

Header always     unset X-Foo
Header onSuccess  set   X-Foo: baz