PubMed Clinical Queries

Hello to all. What is difference between broad and narrow in PubMed Clinical Queries? I searched and considered the details but the strategies were simillar and the just only difference in search box was the change between two words (Narrow and Broad). I remember that in the old interface of pubmed, when i searched a THERAPY query in BROAD, the results were clinical trials however in NARROW, all of results were randomised clinical trials. So it it a big how for me.