deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE
By default means the same thing as "Unless Otherwise Specified" — namely, "unless some other sUtra says otherwise".
Example.
The rule NvultRcau only says "/Nvul and /tRc". What does that mean?
Rule kartarikRt teaches "all /kRt affixes mean the doer by default".
Therefore NvultRcau actually means "/Nvul and /tRc mean the doer".
But the /kRt affix /kta does not mean the doer, because there is a sUtra that says that it means something else.