Formulas – Can conditional formatting refer to another sheet?

I had to check which indices exist on another sheet.
So for the A: A Area, I've made conditional formatting with the following formula:

= COUNTIF (& # 39; other sheet & # 39; A: A; A1)

That had a red bug design.
Then I filled the column X With = & # 39; Other sheet & # 39 ;! A: A and used the formula

= COUNTIF (X: X; A1)

It worked immediately.
What am I doing wrong?