"External schema" means the ECSchema is not stored in the .dgn file. Instead it must be located at run-time, e.g. in some directory on the file system. This is what your 56ff0001 handler ID means. You need to tell ECObjects where to find your ECSchema.
The alternative (and generally preferred approach) is to store the ECSchema inside the .dgn file such that it can always be located. This is what your 56ff0000 handler ID means. ECObjects will find stored ECSchemas automatically by reading them from the .dgn.
I am not familiar enough with V8i ECXAttributes API to make specific suggestions. (Those APIs have changed considerably in the CONNECT edition).
The alternative (and generally preferred approach) is to store the ECSchema inside the .dgn file such that it can always be located. This is what your 56ff0000 handler ID means. ECObjects will find stored ECSchemas automatically by reading them from the .dgn.
I am not familiar enough with V8i ECXAttributes API to make specific suggestions. (Those APIs have changed considerably in the CONNECT edition).