Hi SSHSSH,
As Nathan said, the Host field refers to the HTTP host header as defined in RFC2616 for HTTP 1.1. You can use one or more HTTP classes to select different pools for different classes of HTTP traffic. For example you could have host1.example.com request sent to one ASM policy and a separate pool compared with host2.example.com.
For details on the HTTP class object, you can check these links:
The HTTP Class profile type
http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-concepts-11-1-0/ltm_protocol_profiles.html1210980
sol8018: Overview of the BIG-IP HTTP class traffic flow
http://support.f5.com/kb/en-us/solutions/public/8000/000/sol8018.html?sr=18877158
Aaron