How to select which fields can be merged?

based on this article

www.microsoft.com/.../merge-duplicate-records-for-accounts-contacts-or-leads.aspx

When I open the window of merging two records - I see the list of fields. Some of them are system, some new created.

But there are maybe only 30% of the fields in this window, compared with the number of fields on the entity form. I need some more fields of the entity for merging and don't need some of the proposed.

How can I set which fields will be available on the merge form? How does CRM decide which field to show for merging?