The categories broadly quantify the level of Mackintosh's involvement in a project, ranging from principal designer (Category 1) to no known input (Category 4). It is recognized that the process of developing a building is highly collaborative, ordinarily involving a range of skills, both creative and practical, and that more than one person within the practice would have been involved to varying degrees throughout the evolution and delivery of a project.

Category 1 and 2 projects include detailed comment and a range of illustrations. Category 3 and 4 projects are given more summary treatment, generally confined to Introduction, Drawings, Job Book and Bibliography; this reflects their status in relation to Mackintosh, not their individual importance.

Authorship category 1 Projects identified as designed by Mackintosh by reliable sources during his lifetime; or so distinctive in style that their authorship seems beyond doubt.
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Authorship category 2 Projects for which there is stylistic or documentary evidence that suggests Mackintosh designed a specific but relatively minor part.
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Authorship category 3 Projects for which there is evidence of Mackintosh's involvement but not in a design capacity e.g. the presence of annotations in his handwriting on drawings.
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Authorship category 4 Projects by the office dating from 1889-1913, for which there is no documentary or stylistic evidence of Mackintosh's involvement. Also projects which have been wrongly ascribed to Mackintosh in the past.
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