The investor is usually the person or entity controlling the account.  In the case of some retirement plans, the Investor may be different from the official owner of the investments.  The investments could be owned by a retirement trust.

The Investor is the primary entity in the Advisors Assistant Investment Module.  Investments are keyed to the Investor.

Usually, the Investor will be your client.

Investors' names are associated with accounts.  All of the individual assets in an account will, by definition, be associated with one investor.

You cannot change the investor for just one asset in an account.  You must change it for all assets in an account.