Hi,

Let us write SQL query to understand this:

select s.name,a.name                                 (1) 
from s_evt_act a,                                       (2)
s_srv_req  s                                               (3)
where                                                          (4) 
a.SRA_SR_ID=s.row_id                            (5)

now we look at the ActivityBC:
we can select fields from two or more tables :     see line  (1)
Base Tables:  s_evt_act                                                      (2)
Join definition:
          Join:                   Service request
          Table Name:       s_srv_req                                        (4)

          Join Spec:           a.SRA_SR_ID=s.row_id                (5)

in the same way: 

link will have 1:M relation.
so, link spec (PrimaryKey-ForiegnKey mapping) will have the relation between the two tables (between two BC's pointing fields bought from table level)