Refers to modules that are 3 rack units (U) high – the Eurorack standard, which is by far the most common modular format today, even though it’s one of the youngest formats.
A single rack unit is 1.75” (44 mm); that would lead you to believe that a Eurorack module’s front panel is 5.25” (133 mm) high – but in fact, they are slightly shorter (128.5 mm) to allow room for a small rim or “lip” that exists on some rails. This page on the Doepfer web site – the original champions of the Eurorack format – explains more.
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