The OBS= and FIRSTOBS= are system options used to specify the first and last observations read from input datasets. For instance, if I wanted to process only observations 4 through to 7 in the ecprg193 library and country dataset, I could write the following:

libname ecprg193 '/home/marija/ecprg193';
options firstobs=4 obs=7;
proc print data=ecprg193.country;
run;

OUTPUT:

For more information, see: FIRSTOBS= Data Set Option: https://support.sas.com/documentation/cdl/en/ledsoptsref/63326/HTML/default/viewer.htm#p0wjxoxrco6dsgn1ls5n3mbybcng.htm

OBS= Data Set Option: https://support.sas.com/documentation/cdl/en/ledsoptsref/69751/HTML/default/viewer.htm#p0h5nwbig8mobbn1u0dwtdo0c0a0.htm