I thought if you control 5 (meaby more or less) ordered territoryes you get bonus units (more units if ''continent'' is in middle or it haves a lot of territoryes).
I made and example on Europe map so you can easily understand.

Example-If you control all yellow territoryes you get 4 units becouse there are 4 territoryes but you also get +1 unit becouse you control yellow ''continent''.
*EDIT*
Blue ''continent''- 4 units +1 unit
Red ''continent''- 4 units +1 unit
Yellow ''continent''- 4 units +1 unit
Green ''continent''- 4 units +2 unit
Pink ''continent''- 4 units +2 unit
This is example of bonus units by ''continents'' for Europe map. All ''continents'' on this map get +1 bonus unit except green and pink ''continents'' becouse they are in middle of map. Of course this example of bonus units for Europe map is just my idea/opinion how it ''needs'' to be.