Log:

306  data work.accounting;
307     set work.dept1
308         work.dept2;
309  run;

WARNING: Multiple lengths were specified for the variable jobcode by input data set(s). This may
         cause truncation of data.
NOTE: There were 3 observations read from the data set WORK.DEPT1.
NOTE: There were 3 observations read from the data set WORK.DEPT2.
NOTE: The data set WORK.ACCOUNTING has 6 observations and 1 variables.
NOTE: DATA statement used (Total process time):
      real time           0.05 seconds
      cpu time            0.03 seconds

Commentary:
The dataset WORK.DEPT1 appears first in the SET statement set work.dept1 work.dept2; wherein the SAS compiler assigns a length of 5 for the variable jobcode whose attributes cannot be change further in program. The log above shows a warning message for multiple lengths of the variable jobcode but all the values read from dataset WORK.DEPT2 are truncated to 5 characters. This is solely because of the order in which the datasets appear in the SET statement.

To change attributes of a variable, we would define the attributes first for said variable in the data step (e.g. LENGTH statement) and then proceed with the SET statement.