PLC, Thanks for the information.
Peter,
This reason is confuscated.
1. We are using the new Siemens 22 inch comfort panels for HMIs. TIA portal supports this model, but WinCC Flex does not. Since WinCC Flex is being "phased out", support for the comfort panel will not be added"
2. We are using two high availability S7-400H for the processors. Unfortunately the bright minds at Siemens have not added the capability to setup redundant processors in TIA portal. So for this I am having to use Step 7.
3. So TIA portal HMI software doesn't automatically integrate with Step 7. Thus all tags must be manually typed in/imported. In a effort to simplify a tag naming convention for this situation all HMI tags will be moved to bit memory under a specifc scheme where the numbder in the hundreds place indicates a specific type of IO or location. When you can easily click on a tag it makes it easier not to make a mistake. In this situation is would seem using Mxxx.x makes it easier to keep track of things.