How many composers have written a Sonata No. 3?
Musical works often share title characteristics (form, instrumentation, numbers, keys, opus numbers) with other musical works. Each catalog record is tagged with the fullest combination of these characteristics so that difference forms of a musical work are grouped together (more on this below), but also so that individual works are separated out even if they share titles.
Each score is tagged with its genre or form and other work information (key, opus or catalog no., and instrumentation). If the composer has written more than one work in a particular form, then the form tag is made plural.
For example, try searching for Symphonies instead of Symphony.
The title tags added to each record can be searched. They follow a standard format: FINK (Form, instrumentation, number, and key). For Beethoven's Ninth, that tag reads: Symphonies, no. 9, op. 125, D minor.
Many of the big score publishers are located in Europe (where much of the music in our collection was originally written). This means many scores are published in German, French, Italian, and other European languages in equal measure with English. You may want to find The Marriage of Figaro, but the publisher issued it as Le Nozze di Figaro or Diedes Figaro.
There's a big difference between looking for opera The Marriage of Figaro and looking for the aria "Voi Che Sapete". Songs and movements that are tucked inside of larger works can be tricky to find, especially since they are often published as arrangements in collections (e.g. 24 Italian Songs and Arias).