GuideGuide does a lot of automatic math for you. To do this, it infers what you’re trying to do based on what you have selected. This is called context. There are three different kinds of context, selection, artboard, document. GuideGuide select one context for its action, based on that order.

GuideGuide will always chose the current artboard over the current document, because if artboards exist, it assumes that is the preferred location for guides. If a selection exists, GuideGuide will prefer it above all other contexts. However, the behavior of a selection varies between applications.

Photoshop

GuideGuide will use your artboard by default, if one exists, otherwise it will use the document. If no artboard is selected, GuideGuide will run its action in all artboards. If one or more artboards are selected, it will only run in the selected artboard.

If a marquee selection exists, it will be used.

Illustrator

GuideGuide will use your artboard by default. This is the artboard that contains the last object you interacted with. It is not possible to select multiple artboards, so if you wish to create a grid in more than one artboard, you must run GuideGuide multiple times.

A selection is any object that you have selected.

Sketch

GuideGuide will use your artboard by default, if one exists, otherwise it will use the current Page. A selection can be both objects and artboards, but when mixing them you may experience unexpected results.