The S-20 has six torpedo tubes capable of firing up to 18 torpedoes and missile canisters, which include the 533mm Yu-6 heavy torpedo, naval mines and 300km range YJ-82 AShM. Pakistan is already working to navalize HATF-7 land attack cruise missile for launch from its current Agosta 90B submarines, so the new S-20s would almost certainly also be designed to carry nuclear armed HATF-7 missiles.
This would give Pakistan an additional credible second strike capability by way of,
a) Being able to launch nuclear strikes from previously inaccessible (littoral - shallow depth) areas like the Bay of Bengal or other coastal areas off of the Indian coastline, where the heavier 6000 ton S-30 can't go but lighter 2200 ton S-20's can,
b) And that too with an underwater nuclear deterrent like the nuclear tipped HATF-7 (range of 750-1000km).
So - Pakistan's naval plan calls for twelve submarines. PN currently operates five French Agosta class, with two of its Agosta 70s over 40 years old and in dire need of replacement. My understanding is that the S-20 (Song Class) or S-26 might be the Agosta replacements. And the S-30's (modified type 032 'Qing Class' as some are saying) could be the new upgraded class for the PN as a temporary Indian SSN deterrent.
song class submarines are sitting ducks for ASW. china's submarine technology is quite far away from maturity