there is no Modbus standard that defines the word which encompasses the coil bits?
The Modbus standard does define a format for function code 01, read coils, where the up to 16 coils are bit packed into a 16 bit data word which occupies one Modbus register.
Quote from a Modbus standard, MODBUS Application Protocol Specification V1.1b
----------------------------
6.1 01 (0x01) Read Coils
This function code is used to read from 1 to 2000 contiguous status of coils in a remote device.
The Request PDU specifies the starting address, i.e. the address of the first coil specified, and the number of coils. In the PDU Coils are addressed starting at zero. Therefore coils numbered 1-16 are addressed as 0-15.
The coils in the response message are packed as one coil per bit of the data field. Status is indicated as 1= ON and 0= OFF. The LSB of the first data byte contains the output addressed in the query. The other coils follow toward the high order end of this byte, and from low order to high order in subsequent bytes.
If the returned output quantity is not a multiple of eight, the remaining bits in the final data byte will be padded with zeros (toward the high order end of the byte). The Byte Count field specifies the quantity of complete bytes of data.
---------------------
Unpacking the register word is the task of the slave or Master, depending.
Some PAC's use an entire integer or floating point holding registers to indicate the status of a ooil or discrete input register, which wastes lots of bits but with ethernet comm, the loss isn't terribly noticeable.
I.e. we could use a holding register for example & get the embedded controller to unpack this register to the 16 discrete coils?
If you have the ability in the embedded controller to unpack, then yes, use a holding register and unpack it for the 16 coils.