Here are some guidelines to follow to choose the most appropriate store for placing directory information (usually requested as a schema extension, with additions of attributes and/or classes)

If a schema change is required for an enterprise-wide application, such as Exchange Server that has a major integration (effect’s most objects in the forest) and required interaction with security principals the change probably has to take place in the main forest. However, if the schema change is required for an application that a small population of the organization will use, You should determine whether deploying a such global change to satisfy the needs of this small population of the organization is warranted, in addition you should analyze whether the schema change is a long-term or short-term requirement, also if the data to be hosted in the directory is frequently changing, or is already hosted in a different format with in the forest,

You should consider an alternative directory store, such as a specific application directory using Active Directory Lightweight Directory Services (AD LDS) to support applications that depend on schema extensions that are not desirable in the AD DS directory— for one or more reasons such as schema extensions that only are useful to a single application or only required on a short-term basis.

The following table supports the process to determine the most appropriate directory information store for a particular application/schema extension.

 

Description

Points

A small population of the organization will benefit/use the schema extension, less than 40% 2 points
Schema extension are deployed on short-term basis, application/system lifecycle equal to/or less than 2 years 2 points
Schema extension will host data already available in AD DS 8 points
Schema extension will store more than 256k on a single object 3 points
Schema extension will introduce none-optimizeable LDAP queries 4 points
Schema extension OIDs can’t be verified 12 points

If the schema extension qualifies for more than 3 points above, I advised to choose Active Directory Lightweight Directory Services (AD LDS) as the directory information store over Active Directory Domain Services (AD DS).