In simple terms, using the untag command in your config will cause those ports to be dedicated to that VLAN. Using the tag command will cause that VLAN to flow down that port.
So for example, when you show config:
vlan 1
untagged A1-A24
no untagged B1-B24
vlan 2
untagged B1-B24
tagged C1
So ports A1-A2 are in one VLAN, while ports B1-B24 are in another VLAN. The two VLAN are operating at layer 2, so think of each group of ports are two logically different switches. When you put an IP address assignment to them:
vlan 1
untagged A1-A24
no untagged B1-B24
ip address 10.0.1.10 255.255.255.0
vlan 2
untagged B1-B24
ip address 10.0.2.10 255.255.255.0
tagged C1
It now becomes a layer 3 interface. When you add the command
ip routing
your VLANs will now "talk" (route) to each other (they won't otherwise).
The C1 port used in this example is connecting to another VLAN aware switch. The VLANs being pushed down this wire will be the default VLAN 1 (it's the default VLAN on all switches, so that's why you don't see a tagged command under that interface) and VLAN 2. If you had another VLAN defined, say VLAN 3...as long as I tag C1 under the VLAN 3 interface, then that VLAN will also get pushed down the wire. If you are connecting another switch to the C1 port on the 5400, then you will basically do the same type of config on the other switch, making sure you define the same VLANs and tagging the port used for uplink back to the 5400. Once done, you can divy out ports on that other switch to VLAN 1 or VLAN 2, etc... by untagging them under the VLAN's interface.
Hope that helps.