Shared IP tag: a digital "mark" that helps separate sites within one plan on one logical server. All sites located on the same logical host under the same plan are usually assigned to the same shared IP. This feature allows to give a different shared IP to a group of sites of, say, explicit nature. This may become necessary as many corporate firewalls filter sites based on their IP, not the domain name.
Normally, you are expected to have devoted one IP address for each shared IP tag. To use a shared IP tag, you need to have it defined for every logical host.
Numbers 10 to 99 are acceptable. The default value of 2 is assigned when the field is submitted being empty.