What is this little symbol in the upper left of one board?

[MarieMeyer]

This little symbol has appeared in the upper left corner of my first artboard but none of the others. Possibly the result of the cat walking across the keyboard. What is it?

[Mike_Gondek · Accepted solution]

Slice

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They are used for making webpages, and slicing up a graphic into multiple images when using save for web.