The mailing label field is located in the Family Record and is used in Mailing Labels and by the Church Budget Envelope integration.  When printing labels or envelopes this will be one of four options you can choose for the first line of your label or envelopes. If you use the Church Budget integration function Church Budget will first look in the mailing name field.  If it has data in it, the value in the mailing name field will be the first line on the envelope printing.  If there is nothing in that field then it will default to Salutation + First + Last Name for the first line.