Hi Ali,
In a message box, the ‘$’ character is usually followed by a number (1-9). In Max, this is what is called changeable or replaceable arguments. From the message reference page:
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A dollar sign ($), followed immediately by a number in the range 1-9, is a changeable argument. This argument’s value can be replaced by the corresponding item in a list received in the inlet. (Example: $2 stores the second item in a list as its value before sending out the contents of the message box.) The value of a changeable argument is initially 0.
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In an object like [if] or [expr] (and some others), the ‘$’ can be followed by a data type abbreviation (f(float), s(symbol), i(integer)). These are also considered changeable arguments and generally used as variables in an equation of some sort. Check out the reference entries for these objects for more info.
Hope that is helpful!
-Ben
