Internal transfers are when a student transfers from one of Columbia's schools to another school also at Columbia.
Between CC and SEAS
Students rarely transfer from SEAS to CC, but it's nevertheless possible.
Between CC/SEAS and GS
If you drop out and aren't allowed back as a CC/SEAS student, you may be able to return via GS. There is apparently a procedure for transferring from GS to CC/SEAS but there are no documented cases of this occurring.
Between CC/SEAS and Barnard
Haven't heard of any cases of CC/SEAS transferring to Barnard, but although infrequent, the Barnard to CC/SEAS does occur.