My experience, limited as it is, gives me the feeling that the iMessage service uses any old text, as far as I can see, as an iMessage address / username. _But I may be very wrong._ I don't know what syntactic checks or validation they carry out on the value of an iMessage address. Its value could look like an email address or like a phone number. My own iMessage address looks like an email address but there is no such address in existence.
If it can convert from iMessage to email or to SMS then I am wrong and it must have a 'type' field internally telling it what the iMessage address ‘means’.
iMessage imho tries too hard to 'magically' blend Apple's proprietary (internet) message service with SMS messages. In making it appear magical, I think maybe they make it needlessly hard to understand, and hard to trust. I've never been much for either SMS or messaging, but as far as I have figured out all it really does is, under the skin, one of two things...
1). Apple may 'know' that the recipient is another Apple user. They'd know that because, after checking their servers, they know that the email address, and/or the mobile phone number, have been associated with an iMessage account. In that case, a message is preferably sent normally as native iMessage, over Internet. It does not involve SMS at all, even if phone number was used as address, because Apple know the recipient has an iMessage account.
2). There are times when Internet delivery cannot be used. Maybe the destination is just a phone number that is not known to Apple, or maybe delivery problems arise using the Internet route. In that case, the message will be sent as SMS. It will be sent to the supplied phone number if one was supplied as addressee. If the message was sent using an email address, and that email address is associated with an iMessage account that also has a phone number, it may be sent to that phone number.
The real magical part starts, and this gets really very useful, is when you configure an iPhone to 'forward' text messages for other Apple devices. In that case, I can compose a Message on my iMac desktop, addressing it to a phone number that Apple know nothing about. When I hit 'send', my iMac actually sends it (using iCloud) to my iPhone, which in turn sends it as an SMS message. The recipient is unaware that it started life as anything other than an ordinary, dumb, SMS. Similarly, the iPhone forwards incoming SMS texts so that they can be merged into the iMessage App on all devices.
There are of course various options to enable/disable iMessage, or to inhibit various features, in which case iMessage may become simply the App for sending and receiving SMS texts.