The “stock” LDAP schemas are quite extensive. When you add in the registered schemas created by individual organizations, just about every piece of information that you could possibly imagine can be represented with a schema.
Still, the day might come when you find that you have to record the tail length of a user's pet llama or the number of goldfish that someone can eat in a minute, or, on a more practical note, something important to your business, such as health insurance identification numbers. This is when you'll want to extend the LDAP schema universe.
To understand the process, let's look at two schema entries from the core LDAP set.
attributetype ( 184.108.40.206 NAME 'description' EQUALITY caseIgnoreMatch SUBSTR ...