Recommendation to relate 1 form with another?

Recommendation to relate 1 form with another?

I have a form, Policy.  On it I have field Policy Number and Expiration Date.  I have another form, Attachments, since a Policy form can have multiple attachments related to it.  One attachment is only relevant to one policy.  On Attachments, I have fields, Title and File Upload.

What is the recommended relationship direction and why?

1) Put a lookup field on Policy to lookup the Attachment title (multiselect) or
2) Put a lookup field on Attachment to lookup the Policy number