1) Left-to-right mode selects only shapes that that completely inside
   the selection frame.
2) Right-to-left mode selects the shapes that are crossed by the frame.
3) Blue frame means "covering" (or "containing") selection
4) Green frame means "crossing" selection
5) TODO: make two cursors/icons for the two modes, so people could
         understand it without reading the manual.

See point 4A in the requirements for the details:
https://phabricator.kde.org/w/krita/vector_tool_requirements/

CC:kimageshop@kde.org