The note after your data step:
NOTE: Variable StateCd is uninitialized.
Means at that point there is NO VALUE for the StateCd variable.
Running this code:
PROC FORMAT;
VALUE $StateCd
'IA' = 'Iowa'
'MS' = 'Mississippi'
'UT' = 'Utah';
RUN;
data junk;
input x $;
datalines;
IA
BC
MS
UT
.
;
ods listing;
Proc print data=junk;
format x $statecd.;
run;
Yields this output.
Obs x
1 Iowa
2 BC
3 Mississippi
4 Utah
5
So a "blank" or missing value value with that format does NOT create "Unknown" for output. So, either you have a different actual format ACTIVE than you have shown or something else is happening.
Most likely the actual Format code you ran had either a missing or "other" assigning "Unknown" and that makes sense because you have no actual values for the variable.
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