TYPO3 Core

Line 268 sets TYPO3 to production mode no matter what is definied above.
Example:
you define a local environment with a local domain to development you need to

a) disable line 268

or
b) enable line 267 and define a production server

to make your local environment running in development.

As production is the php-default anyway, line 268 should stay where it is but should be commented.
So it stays there as an example and nothing really changes except you dont need to change this line if you want to run a local install as development.