Weight statement can be used to exclude observation from the statistical procedure. If Weight statement is not specified, each obs has a default weight of 1.
PROC MEANS
0: Counts the obs in number of observations
<0: convert to 0 and treat like 0
missing: exclude
PROC FREQ
0: ignore obs unless specify the ZEROS option, which include obswith 0 weights.
missing: exclude
PROC TTEST
0: exclude
<0: exlude
missing: exclude
Basically, assigning a missing value to the weight field to exclude the observation and assigning 1 to the weight field to include the observation in the analysis.