Mi a különbség a multiple és single html között?

Egy angol szöveget fordítok és ezt a dolgot nem igazán tudom értelmezni, hogy mit jelent.

Itt van a szövegkörnyezetben:

"In the early 1990s, web site statistics consisted primarily of counting the number of client requests (or hits) made to the

web server. This was a reasonable method initially, since each web site often consisted of a single HTML file. However,

with the introduction of images in HTML, and web sites that spanned multiple HTML files, this count became less useful. The first true commercial Log Analyzer was released by IPRO in 1994."