Templates
A template is a document that acts as a starting point for new documents. For example, the "A4 with Grid (Metric)" template that comes with OmniGraffle has its canvas size set up for printing to A4 paper, and has a grid with .5cm squares. OmniGraffle comes with several templates, and you can create your own if there are certain canvas arrangements you use often.
When you save an OmniGraffle document as a template, everything about it is preserved, from the canvas size and grid layout, to layers, to visibility of extras, to the automatic layout settings. Once you have everything looking exactly the way you want it, select Save As Template... from the File menu. The default save location is inside your own Application Support folder, but you can save the template in any of the paths listed in the Templates preferences. Once a template is saved in one of these paths, it appears in the New From Template submenu of the File menu, and becomes eligible to be the default document template in the Templates preferences.
When you create a new document, it starts out looking exactly like the default document template. This is unless you create the new document using the New From Template command, in which case the new document looks like the template you select.