For example, I work with a "3D" instance of MP:

\defineMPinstance
 [three]
 [format=metafun,
  extensions=yes,
  initializations=yes,
  method=double]

\startMPdefinitions{three}
 input three ;
\stopMPdefinitions

Where three.mp contains my 3D projection macros.

Use is then

\startMPcode{three}
...
\stopMPcode

Does this mean that \startMPdefinitions is identical (in effect) with \startMPextensions? Its use here seems to suggest.

Or if otherwise, what is the difference in usage?

Hans van der Meer