What does the following warning mean?
ptiitt5402m0000( 743): Warning: External 'tiitt502.kmefec' passed as reference to function 'tiittcalcul'.
I want to understand if it is something to fix, and how.
Thanks

No need to fix but to be aware of... It is a warning telling you this variable gets changed in this function call, but others may read that value within your bshell. Other sessions / form processes / reports etc access that variable under the same name and in your function that value is possibly changing. This may corrupt or influence those other users of that (extern) variable.