One of the many requirements of webcasters is that they don’t exceed the Sound Recording Performance Complement, which is defined in the law as…

(7) The `sound recording performance complement' is the
        transmission during any 3-hour period, on a particular channel
        used by a transmitting entity, of no more than--
                    ``(A) 3 different selections of sound recordings
                from any one phonorecord lawfully distributed for public
                performance or sale in the United States, if no more
                than 2 such selections are transmitted consecutively; or
                    ``(B) 4 different selections of sound recordings--
                          ``(i) by the same featured recording artist;
                          ``(ii) from any set or compilation of
                      phonorecords lawfully distributed together as a
                      unit for public performance or sale in the United
                if no more than three such selections are transmitted
        Provided, That the transmission of selections in excess of the
        numerical limits provided for in clauses (A) and (B) from
        multiple phonorecords shall nonetheless qualify as a sound
        recording performance complement if the programming of the
        multiple phonorecords was not willfully intended to avoid the
        numerical limitations prescribed in such clauses.

Our short version of this is that in a three-hour period the station can transmit no more than:

    1) three songs from one album or compact disc, if no more than two of these are played in a row; or
    2) four songs by the same featured artist or from the same box set, if no more than three of these are played in a row.