How does "Server Generated" differ from NULL?

IDS 12.10.FC3 Solaris 10 1/13 A table in our production system has a SERIAL column. So, I think it can't be NULL. But, if I execute the following: "SELECT * FROM <table> WHERE <serial_col> IS NULL, I get a couple thousand rows in which that column seems, effectively, to be NULL. If I use dbaccess, it displays as just white space. If I use Server Studio, it displays as greyed out "Server Generated" (not greyed out "NULL", as one usually sees in Server Studio). And, it appears to function as if it were NULL. How does "Server Generated" happen, and how is it different from NULL? BTW, it breaks dbimport. I can dbexport that database (including that table), but dbimport blows up when it encounters it during the import process. DG