As well as contacts that are applicable to a client, you may have contacts that are applicable to just one activity for a client. For example, if a client has a fellow resident approach you on their behalf as someone able to substantiate your client's claims, you may like to enter that person as a contact, against the activity rather than against the client.
This can be achieved in the Details tab of the Service History tab, using the hyperlinks provided for the issue:
The process for entering contacts against activities is exactly the same as described for Adding/Editing client contacts & details.