How to use Custom template variable %RetainerAppliedToInvoice%

How to use Custom template variable %RetainerAppliedToInvoice%

Hi,

I'm creating a custom retainer invoice template. I like to display the list of past linked invoice, and the %RetainerAppliedToInvoice% seems to be the variable to use. But the default custm template doesn't use it.

Is this varaible juste a texte list of pas retainer invoice, or is like the itemlist, a variable in a loop ? How to use this in the template ?

Thank you