Hi, Cindy. Welcome!
The post you reference pertained to Instant Text, (IT), a very popular expander program here. Although I don't know which particular post you read, I can tell you that in Instant Text, one cannot start an abbreviation (also often called short or short form) with a number. For instance, say you have an entry for 5-FU, your abbreviation could not start with 5, but would have to start with a letter. If you import a large amount of entries from another program or AutoCorrect, Instant Text will automatically place a certain letter ahead of the digit for any entries starting with a digit. In the end, having always the same letter in front of a number makes it easy to remember, of course. Shorthand, another popular program, does allow you to start a short form with a digit or other non-letter character.
"Phrase side" and "word side" are also features of Instant Text.
Since you are not yet using any expander, I suggest simply reading up on the different ones. Here's a
post to start you off. You can peruse the board and make specific searches to learn more. Also, as far as where to start on your system for abbreviations, again, you might read up here in this very section of the board for ideas. The main idea for many folks is to have a "system" such that, you would not actually have to remember all of your shorts (an impossible thing for any human), because your "system" would tell you what it is. For instance, say you have a system for all words ending in "itis." Well, take mine, LOL. It's the first three letters of the word and then the letters "it." So phlebitis would be phlit and tendinitis would be tenit. Nothing to remember except the system itself, you see? Now, to me, when you combine systems with an expander that gives you visuals, such as both Instant Text and Shorthand, you have the best situation of all. On the other hand, for those who don't care for systems and also know they can't remember all their shorts, Instant Text is excellent for that and Shorthand, if one uses the suggestion window, will also work for those folks.
Hope this is helpful.