Loibl Johann writes:

Could someone tell me where is the difference between URL0 and BASE3 or URL1 and BASE2 ... URLn removes "n" URL path components from the right, BASEn consists of the first "n+1" or "n+2" URL components.

Thus, if you URL is "host/u1/u2/..../un", then URL0 is URL, URL1 is the last component "un" removed, ULR2 is the last two components "u{n-1}/un" removed etc. Usually (unless you access Zope via CGI), BASE0=BASE1=host. BASE2 is "host/u1", BASE3 is "host/u1/u2" etc. If your URL has 3 components, then URL0 and BASE3 are identical, otherwise, they are not. Dieter