What do numbers in parenthesis after elements respresent? I have seen elements with a number in parentheses after them in literature, but cannot find what it means. What does the number in parentheses after an element represent, such as in W(100) or Mo(110)?

https://aip.scitation.org/doi/abs/10.1063/1.1727919
https://www.sciencedirect.com/science/article/pii/0039602880900886
These are called Miller indices and they identify a particular surface of a crystal.  (You can form different faces by cutting the crystal in different ways.)  When you see something like "Mo(110)" it means "the 110 surface of a crystal of molybdenum."