Do you not get the sense, even slightly, that there is something oxymoronic about... "not all PLC allow data register addressing on bit level."
If you reference a particular bit in a byte, word, double-word etc., then you are simply referencing a particular "room" in a particular "hotel"... at address such-n-such.
In human-speak, you might reference first the Room Number, then the Name of the Hotel, or you might reference first, the Name of the Hotel, then the Room Number.
As in... "See if someone is in Room 12, at the Hilton on 3rd."
Or... "See if someone is at the Hilton on 3rd, in Room 12."
Now, in this crazy world of computers... if you (the processor) want to access a particular "room" in a particular "hotel", you reference first the "hotel", then the "room". That is, first you specify the address of the Hotel, then you specify the Room Number.
But wait... there's even more craziness...
In computers... the moutain DOES come to Mohammed!
If you want to see if anyone is in Room 12, at the Hilton on 3rd... you do NOT go to the "hotel"!
Instead, you bring the "hotel" to you (the processor)! And you place it in a real, physical place, where you can even touch it (at least you could in the old days before processors were compressed onto single chips). That place is called a "Register".
Once the contents of the "hotel" is loaded into the "register", you can access any room in the register, really, you can.
Now, as Sergei said, some PLCs don't allow you to just walk into any room at the hotel to see if someone is there... however, there are ways.
You can use a Mask (like a crook, maybe?) and extract a particular room from a hotel. Then you force the occupant, if any (or at least a copy of him, if any), to move to the same room number in a different, empty hotel. You can then evaluate the contents of the "empty" hotel to see if anyone is there. If the hotel shows empty, then the original room in the original hotel is unoccupied. If the "empty" hotel is no longer empty, then the original room in the original hotel is occupied!
At that point, that information is available to anyone, as long as that information is not wiped.
Yeah... it might be a cute story, but the point is, strictly speaking, the address of the Hotel is NOT a register. It is simply the address of the hotel. The contents of any hotel can not be examined until the hotel is "brought" to a "register" where the contents can be examined directly, or otherwise operated on.
If you want to call the address of the hotel a register, fine... live in ignorance and bliss... all is happy... whether you really understand what is happening, or not.
To call the address of the hotel a register hides what is really happening... like in the Matrix... things ain't necessarily what they seem!
What's really happening?
When you KNOW what's really happening, you KNOW how to manipulate what happens!
Seems like a small (semantic) difference to some... but the ramifications are significant!