In my opinion the French is suffering from management speak. What it really means is that the particular group of customers affected by a major failure of service will receive compensation. To call this group of customers an 'external file' is plain weird, mangling specific IT terminology to 'sound' impressive. Of course the customers are 'externe' - all customers are. It's your decision of course whether or not to stick with the management speak style or go for something more direct.

PS "fichier externe de clients" get zero google hits quite often a sign that this is not a 'normal' expression.