What does it mean when SITE_NUMBER is blank in IRC_SOURCE_TRACKING table?

Summary:

SITE_NUMBER blank on IRC_SOURCE_TRACKING

Content (required):

We noticed that SITE_NUMBER of some data from IRC_SOURCE_TRACKING table are blank. Anyone knows why this field is blank on this table?

Our query is returning blank as we are trying to join this table directly to IRC_CX_SITES_VL using SITE_NUMBER.

Code Snippet (add any code snippets that support your topic, if applicable):

SELECT 
TRC.CANDIDATE_NUMBER, 
SITB.SITE_CODE, 
SITB.SITE_NAME
FROM IRC_SOURCE_TRACKING TRC, IRC_CX_SITES_VL SITB
WHERE TRC.SITE_NUMBER = SITB.SITE_NUMBER