Janab,
It is not me but denso saying it, kindly check their website, they have specified as per the manufacturer requirements.Check out from here for the pakistan model swift, M13A RS413:
DENSO SPARK PLUGS
Another point is that as per owner manual of swift recommended iridium plug for swift is IFR6J11 means J series and Denso has themselves mentioned that corresponding to NGK J series they have 20(i.e IK20) and IK16 is corresponding to NGK 5 series.Check it here:
DENSO SPARK PLUGS - Basic Knowledge
And IK20 and IK16 have different applications and specifications, you can check from above link.
Hope it clarifies everything:).
For your convenience i am copying the table from denso global website:
DENSO 16 20 22 24 27 29 31 32 34 35
NGK 5 6 7 8 9 9.5 10 10.5 11 11.5
CHAMPION 12,11 10,9 8,7 6,63,61 4,59 57 55 53
BOSCH 8 7,6 5 4 3 2