This page announced updates in Stata 11. See a complete overview of all of Stata's data management features.
. merge 1:1 subjid using filename, assert(match)means that you expect all the observations in both datasets to match each other, whereas
. merge 1:1 subjid using filename, assert(match using) keep(match)specifies that you expect each observation to either match or be solely from the using data and, assuming that is true, you want to keep only the matches.