Custom grid locations for both Sketch or Section are positioned ( mapped onto) an 'Origin' which may be inserted within a sketch / section plane operation or at other times.
In Sketching /Section mode, an Inserted 'Origin' is placed by selecting 2D grid positions or part geometry on the Sketch / Section plane only. The axes of the origin are aligned with the Sketch / Section XY axes.
An Origin may be moved / orientated using the move tool. In 2D mode ( Sketch / Section), only XY and rotate about Z are available.
Origins can be renamed for ease of identification and created or moved into Components thus keeping their relative position / orientations with part or assemblies upon a change of Component position.
In 3D mode, Origin placement auto aligns upon geometry selection and all geometry or combinations of geometry can be selected. Standard 3D move operations can reposition Origins as desired / necessary.
The 'Origin' icon is located within the 'Insert' functions / operations.
How to use in Sketch or Sections creation:
Invoke Sketch or Section plane command ( e.g. shortcut key 'K' or 'X' respectively) , either hover cursor over Origin icon in the graphics window ( becoming highlighted ) or hover over Origin /Origins in Structure Browser ( again, highlighted in graphics window) and select. Note the dynamic Sketch / Section plane snap preview and highlighted XYZ Origin axes.
Origin location examples:
GIF example 1 showing Sketch Grid placement with re-position / orientation and new Origin placement.
Gif example 2 showing Sketch grid custom position using an Origin.
Gif example 3. Origins not displayed are nonetheless available for selection - hover mouse to highlight, then select. Note the Plane Toolbar is used to select another plane, orient plane and change to 3D mode.