Instr is the Go Fish function. You can use it to check for the presence of a set of characters in another set of characters. The case for this is generally wanting to see if a another field contains a certain set of characters.
One example of this is when you have an outline code of some kind that is associating a task with some sort of breakdown structure. You want to mark tasks that are NOT at the top level of the breakdown structure. The outline code uses a period for the separator so the values look like this: ABC or ABC.123. So you want to know if a task has a value AFTER the first separator. To do this you just need to know if the field contains a period character. Instr to the rescue.
Add this formula to a Flag field that you call something like “SubValue?”: Instr([OutlineCode1],”.”)
Now SubValue? will equal Yes for any task that contains a period in the OutlineCode1 field and No for those tasks that do not contain a period.
Intr makes it easy to search fields for specific values. In this case it was a period but it could be whole words. You might want to search the Notes field and if it contains certain words you could flag the task. Search for yourself…