I want to know what the UUID equals, since leaving it only in quotes ("") tells me that it can not be null. I'm trying to put it in the following way:
cfdi.CfdiRelacionados = new ComprobanteCfdiRelacionados();
cfdi.CfdiRelacionados.CfdiRelacionado = new ComprobanteCfdiRelacionadosCfdiRelacionado[1];
cfdi.CfdiRelacionados.CfdiRelacionado[0].UUID = ? ;
It's forced to carry a UUID. The information that @AlejandroMedina provides is erroneous, because in that section it is not being used to refer to the documents that have been paid, for that is the proof of payments.
Now, the simplest case I can mention is when you generate a credit note (type = income) on an invoice (type = income), in that case, the credit note must specify the UUID of the invoice on which will apply the accounting effect.