Its adds a level of data security to the application.
Lets say your section access is written as follows. If you had 'strict exclusion' disabled (unchecked), then user3 could see all the values of REGION. If you had it enabled (checked) they could not even access the document because there is no explicit value in the table that they have access to. It adds a requirement to be explicit with who can see which values.
LOAD * INLINE [
ACCESS, USERID, REGION
USER, USER1, NORTH AMERICA
USER, USER2, *
If Strict Exclusion is disabled, all users that have a Section Access entry but do not have data linked to their Section Access entry (data reduction would reduce to nothing) would get access to the document and ALL data (= no data reduction in reality).
If Strict Exclusion is enabled, all users that have a Section Access entry but do not have data linked to their Section Access entry (data reduction would reduce to nothing) will be thrown out of the document (Access Denied)
Only useful/operational when "Initial Data Reduction based on section access" is activated.