National vs Regional vs Institutional conference
I don't know that there are any hard and fast rules for this - I see it as referring to the organization that hosts the conference. Is it a international or national organization or a regional/provincial/local branch of an international or national organization.

Institutional conference = hosted by an institution and attended mostly by people from one institution.

Regional = generally a regional division of a national organization. For example, the American Psychological Association has different regions, which hold their own conferences. See http://www.apa.org/about/apa/organizations/regionals.aspx

Provincial = similar to regional, but when the national organization is subdivided into provinces.

National conference = hosted by a national professional organization.

International = hosted by an international professional organization.

The name of the conference doesn't necessarily indicate if it is a national, regional, provincial, local, or institutional conference. For example, a national conference might be held at the same location every year, and thus might have the name of that location in its title, but it is still a national conference.

Good luck with your scholarship!