I don't see anything about a tag in the docs: https://iota.readme.io/v1.0/docs/transfer
So a tag just get's appended to the message field ?
If a machine uses some kind of tag, it still needs to be sending a message to a receiving address.
Even if all the machines send themselves a message (I think sending a message from address A to address A is allowed) with a tag/message containing the "Identity" of a machine:
An application to view this data needs to somehow parse through the entire set of All transactions in the entire tangle(looking for key tags), and it does not have a set of transaction hashes to search with:
In order to find the "Identities"/tag/message from the machines.
To me, it seems simpler if they send this to a unified public account. It makes it simpler for developers to build applications around connecting to this public account to gather 'tags'/'messages' from machines from a specific end-point (a public account/seed). If all the machines would send an initial message to such an account with it's own identity information (location, purpose etc) upon initialization in the network, it leads to the next concept which allows for a:
Directory of Machines (I called it: iotadir)