Hello @kevin_coulibaly ,
thank you very much for the tips, I have placed it as a service function. It works just fine.
In Service of ContentType A
module.exports = {
...
async filterObject(obj) {
...
}
...
};
Calling in Controller of ContentType X:
let res = await strapi.services.a.filterObject(obj);
I still have to find out how to use it in “config/functions” but for the first time the problem is solved. 
Best regards! 