Many thanks to Prasanth Chaduvula for putting this down in writing:
CSS units are used to style the HTML elements by specifying the size, position, and other properties of HTML elements. There are several types of CSS units like rem, em, px, vh, vw, percentage and each with its own specific use case.
CSS units can be broadly categorized into two main types: absolute units and relative units. Each type has specific use cases and characteristics.
Dave5 
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