The sysopt ipsec pl-compatible is a legacy command that used to be used for VPN creation on older versions of the PIX.
This command basically allowed VPN's to terminate on a PIX without being affected by ASA and NAT
It used link commands to create an encrypted tunnel between two PIXs
The difference between using a link tunnel and IPSEC is the way the tunnel is formed. IPSEC is formed from source interface to destination interface and link is formed from destination interface to source interface.
HTH
Paddy