S_ACT_CONTACT : Description

This table associates a contact with its activities. This relation is suported by column con_id (par_row_id from s_contact) and activity_id (activity_uid from s_evt_act).
This is a one-to-many relationship : A contact can have more than one activity associated, but a position can only be related with one contact.
Keep in mind that con_id is foreign key from s_party and activity_id from s_evt_act.

S_POSTN_CON : Description

This table associates a contact with positions that can acess it. This relation is suported by column con_id (par_row_id from s_contact) and postn_id (row_id from s_postn).
This is a many-to-many relationship : A contact can have more than one position associated, and a position can be related with multiple contacts.
Keep in mind that con_id is foreign key from s_party.

How to relate a contact whit their accounts

Contact accounts can be found in S_PARTY_PER intersection table. This table relates person_id column (par_row_id from s_contact) with party_id column (par_row_id from s_org_ext). Both person_id and party_id are foreign keys from s_party.