The '&' operator is used to specify that you are passing the address of a variable, so that called function can operate on it. It is used to pass a variable into which the function can store a value for the caller. It is a basic concept in C language: if you are unfamiliar with it you should spend some time learning this language, as CVI is pure ANSI C language with some additional controls and libraries.
I suppose there is a typo in your message: SetCtrlAttribute () is used to Set the attribute of a control on the user interface; in this extent it usually does not require the value to be passed as a pointer but as a value instead. On the other hand, GetCtrlAttribute () is used to retrieve the value of an attribute: this is the typical function thet requires the address of a variable where to store the retrieved value, hence the use of '&'.
Sorry for answering in English: I don't know French! On the other hand, consider that this is an international board where normally English is the standard language. NI offers a specific board for french-speaking users that you may want to post to if you don't want to write in English.
