CMYK Guides contain only colours which exist within CMYK colour space, 2868 colour variants with the CMYK percentages to enable confident use of 4-colour process colours in offset lithographic printing. If you only work with CMYK colours your clients can select colours from these guides with confidence that they can be accurately reproduced. This is a different system to the PANTONE PMS system and so it does not relate to Pantone Spot colours. If your clients select Pantone Spot colours and you are required to check the CMYK versions of these, you will need to purchase a Color Bridge Guide.
Color Bridge Guides have squares of the Pantone Spot colours with their RGB and HTML code and alongside are squares of the closest match to the spot colour using CMYK inks and the CMYK percentages.