grammar - Inverted sentence/ reduced relative clause?

My student once asked me about one sentence he read on one particular site. It says " Attached is the receipt." He asked about what kind of grammar is used in the sentence. I am not sure if it is really an inverted sentence, but while surfing the net for answers, I accidentally saw something that could help me figure it out.
One of the commenters in a much closer topic with mine said that it is what we call reduced relative clause. Please help me.