The PANTONE Matching System (PMS) is the dominant spot color printing system in the United States. Printers use a special mix of ink to achieve the color needed. Each spot color in the PANTONE system is assigned a name or a number. There are over one thousand PANTONE spot colors available.
Are PANTONE 624 U, PANTONE 624 C, PANTONE 624 M the same color? Yes and No. While PANTONE 624 is the same ink formula (a shade of green), the letters that follow it represent the apparent color of that ink mix when printed on different types of paper.
The letter suffixes of U, C, and M tell you how that particular color will appear on uncoated, coated, and matte finish papers, respectively. The coating and finish of the paper affects the apparent color of the printed ink even though each lettered version uses the same formula.
In Illustrator, 624 U, 624 C, and 624 M look exactly the same and have the same CMYK percentages applied to them. The only way to truly tell the difference between these colors is to look at an actual PANTONE swatch book.
PANTONE swatch books (printed samples of ink) come in uncoated, coated, and matte finishes. You can use these swatch books or color guides to see what the actual spot color looks like on the different finished papers.