Before selecting an alias level, or a specific multilevel alias, consider the following:

1. User catalog names perform like aliases. For example, if you have a catalog USER.ICFUCAT1 with the alias search level set at 2, then all data sets beginning with USER.ICFUCAT1 are cataloged in this catalog.
2. When defining an SMS-managed data set with multiple components, like an SMS-managed catalog key-sequenced data set, if a user specifies a component name that resolves to a different catalog than the data set itself, the definition fails.
3. When defining a catalog cluster or a generation data group, if the name of the cluster or generation data group matches an existing alias or user catalog name, the definition fails with a "duplicate name" error. This is to prevent the data or index component of the VSAM data set, or a generation data set, from becoming inaccessible.

For example, consider a situation where two user catalogs, ICFUCAT1 and ICFUCAT2, each have an alias. The alias search level is set at 3. There are two aliases defined:

1. Alias A.B for catalog ICFUCAT1
2. Alias A.B.C for catalog ICFUCAT2

If you define an entry-sequenced data set named A.B.C without specifying the data component name:

1. The cluster entry is in ICFUCAT1
2. The generated data component name is A.B.C.DATA

Since A.B.C is an alias for ICFUCAT2, any references to A.B.C.DATA are oriented to ICFUCAT2. Since A.B.C is cataloged in ICFUCAT1, any request for the data component results in a "data set not found" error.

Also, if you have a generation data group named A.B.C, it points to ICFUCAT1. However, the names of the generation data sets are in the form of A.B.C.GxxxxVyy. Even though the generation data sets are actually cataloged in ICFUCAT1 because its base is in ICFUCAT1, later references to any generation data set results in orientation to ICFUCAT2 and a "data set not found" error.

This problem can be avoided if no VSAM data set or generation data group is defined with the same name as an existing alias or catalog. It is your responsibility to ensure that when you add an alias to the catalog, you do not cause existing data sets to become inaccessible.