ADDITIONAL section - what kind of records can we find here?

Solution 1:

As far as I know the standards do not set an a priori limit to only A or AAAA resource record (RR) types in the additional section, in theory any type is allowed.

But processing only occurs when the requested resource record is of a type that allows for a response in the additional section RFC 3597 :

§8 ... Unknown RR types cause no additional section processing. Future RR type specifications MAY specify type-specific additional section processing rules, but any such processing MUST be optional as it can only be performed by servers for which the RR type in case is known. ...

An old overview of which RR types allow for an additional section and what is expected there is RFC 1035

3.3.9. MX RDATA format
...
MX records cause type A additional section processing for the host
specified by EXCHANGE.  The use of MX RRs is explained in detail in
[RFC-974].

versus other records that state cause no additional section processing.

(That RFC is obviously from before IPv6 and AAAA records were defined but that should give an idea of which RR allow for an additional section).

But a fairly obscure RR type that as far as I know has been proposed to be included in the additional section is an RFC 6698 TLSA record.

;; ANSWER SECTION:
    www.example.com.        86400   IN  A   192.168.1.1
    
;; ADDITIONAL SECTION:
    _443._tcp.www.example.com. IN TLSA (1 1 2 92003ba34942dc7415... )