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How many time did you start editing an $identifier or a command, forget about the syntax, check it, and get back to the code? Too many time? This is perfect for you then!
A calltip is a tooltip poping up when you enter into an $identifier() while typing it's '(' or when using auto completion. It displays the syntax of that $identifier with a little description, it even highlights which parameter you are editing if there are more than one syntax for the same identifier, you can go through them using the up/down arrow key (or by clicking on the arrow on the calltip itself), you can make it disappear by hitting the escape key.