In the Composition Editor, there is a Play Mode toggle button. Click to access a drop-down menu and to set Play mode or to review the current mode. Play Mode provides “recommended” settings for both Functional (General) and Load (Load) testing. Pick Custom to enter Custom Mode using your own Composition settings.
Play modes are a shorthand for sets of Composition properties suitable for certain types of testing. In the screenshot on the right, the play mode is set to General. Changing one or more settings manually defines a "custom" play mode.