VIP type of Forwarding (IP) with fast layer 4 profile

This article explains it:

https://support.f5.com/csp/article/K7595

An IP forwarding virtual server accepts traffic that matches the virtual server address and forwards it to the destination IP address that is specified in the request rather than load balancing the traffic to a pool. Address translation is disabled when you create an IP forwarding virtual server, leaving the destination address in the packet unchanged. When creating an IP forwarding virtual server, as with all virtual servers, you can create either a host IP forwarding virtual server, which forwards traffic for a single host address, or a network IP forwarding virtual server, which forwards traffic for a subnet.

IP Forwarding Virtual Servers act more like routers than a typical virtual server. They forward requests to the destination specified in the request, as opposed to sending the traffic to a pool member.

See also this post:

https://devcentral.f5.com/questions/usage-cases-for-forwarding-ip-virtual-server-forwarding-layer-2-virtual-server-53124

Fast layer 4 has minimum profiles and general idea is to fast forward traffic on Layer 4 info only.

Forwarding virtual servers are used when you need you big-ip to route traffic as router would do (more or less). The configuration is simple, the destination is actually the final destination of the traffic. When an IP packet hits the VS, if the destination match, the big-ip will forward the traffic based on the information of its route table. Here there is no load balanced whatsoever, no IP destination IP change, just routing. It still has some peculiarities by default, for example is still statefull, only allowed flows can go through.

You can configure a fastl4 profile attached to the virtual server and enable loose initiation and loose close and make the virtual server stateless (purely a router). Keep in mind the vlans where you enable the virtual server, there is where you are enabling routing.

I don't understand your second question. If you want to make a virtual service available you don't need a forward virtual server for that, it is easier if you just configure the virtual server to listen in the vlan where the traffic reach your big-ip.