Mapped IP (MIP) is a direct one-to-one mapping of one IP address to another(public ip to private ip ) that means hiding some resources (server) from the public.
A virtual IP (VIP) address maps traffic received at one IP address to another address
based on the destination port number in the TCP or UDP segment header. For
An HTTP packet destined for 126.96.36.199:80 (that is, IP address 188.8.131.52 and port 80)
might get mapped to a webserver at 10.1.1.10.
An FTP packet destined for 184.108.40.206:21 might get mapped to an FTP server at
An SMTP packet destined for 220.127.116.11:25 might get mapped to a mail server at
The destination IP addresses are the same. The destination port numbers determine the host to which the security device forwards traffic.
you can refer to Concepts and Examples Screen OS Reference Guide in Juniper web site