When I checked the BOL i found that there is a line stating:

If a table name is aliased in the FROM clause, only the alias name can be used to qualify its columns in the ORDER BY clause.

I did some modification in the query and try to run following scenarios and these all are working.

select a ss from test t

order by ss

select t.a ss from test t

order by a

select a from test t

order by a

Can anyone tell me what does that BOL line means?