Question

Difference between frontiers and boundaries

Answer 1

Border is talking about a specific edge of the country or an area. Something that can be clearly marked. It’s usually an official property line that marks where one country/area begins and another ends. 

The US border with Canada is huge.

Frontier is something more general. It is not a specific line, but an area. It is usually an area or a district near the boarder line. 

Soldiers were guarding the frontier near France. 

Frontier can also be used to mean a certainlimit. 
 
frontiers of knowledge, frontiers of mathematics, etc. 

I hope it makes much more sense now.