The starting point is always an intention. This may be to transfer information but is often to produce action, eg giving an command or placing an order. In either case the sender needs to know what the intended recipient already knows about the topic.
For instance, if I wish to explain collaterised debt obligations to you I need to know how much you already know about markets and derivatives. Similarly, if I wish to order a garden hammock from you I should first establish that you sell such hammocks. At the pragmatic level this provides the shared context within which communication will occur.
(Human beings routinely negotiate these issues but IT systems lack this flexibility. Therefore, where the sender and recipient are IT applications, such as an ERP or online order taking system, these matters must, unless relevant standards already exist, be negotiated between the parties. The pragmatic context may include agreements on the legal significance of messages, the speed with which orders should be honoured, etc. At least some of the context may be stated in commercial terms and conditions, in contracts or in national law.)
From the context and his intent the sender must decide what information he needs to transfer. In the general case of inter-personal communication, including negotiating, teaching, etc., this is, in fact, the most difficult step. In IT systems its often strongly constrained by obvious needs, eg to specify the product being ordered, and constraints, eg everything has to be typed by a telesales agent.
This information is now passed to the semantic level where the sender selects the natural language and/or coding system to be used. The natural language should, obviously, be one that the recipient understands and this also applies to any technical terms used. In the case of coding systems there is often an obvious choice, eg the Gregorian calendar for dates, WGS 84 for lattitude and longitude, but other systems remain in use so it may be necessary to specify the system in use. The choice of units of measurement is also part of the semantic level.
(Where the sender and recipient are IT applications these decisions may be taken by the designer of one or the other or negotiated between them. They will not generally be taken by any individual user. They may be documented in a data dictionary but as natural language text as there are no commonly used notations for expressing semantic choices.)
At lexical level the words are converted into letters and punctuation marks. In most cases the lexical rules
At coding level the symbols are converted into bytes (this is almost always automatic).
Finally, the bytes are inserted into the fields in a pre-agreed structure (the format level).
(Where the sender and recipient are IT applications the format may be decided by the designer of one or the other or negotiated between them. There are several commonly used notations for defining such structures. It may be documented in a data dictionary and also stored in a database schema.)
We now have a sequence of bytes that can be sent electronically as a message with confidence that the recipient will be able to recover the intended meaning.