Before using an abbreviation for the first time in a document, write out the words or name in full and introduce the abbreviation within parentheses. After that, use the abbreviation alone
The Alberta Energy Regulator (AER) is based in Calgary. The AER also has nine field centres.
This also applies to units of measure.
Pipeline A was 5 kilometres (km) long. Pipeline B was 10 km long.
Always redefine abbreviations in executive summaries, appendices, tables, figures, and captions. These elements are often read in isolation from the rest of the document, and redefining the abbreviation makes it easier for the reader. In some contexts (like long, modular directives or manuals), we may recommend redefining abbreviations in every top-level section.
If the first time you define an abbreviation happens to be a possessive, first try to rewrite it. If that’s absolutely not possible, then the ’s must also appear with the abbreviation.
Acceptable: The site is within the wildlife management unit’s (WMU’s) boundaries.
Preferred: The site is within the boundaries of the wildlife management unit (WMU).
You do not need to formally define obvious abbreviations or truncations of a name or title.
Directive 020: Well Abandonment applies to all these situations. However, Directive 020 does not apply if…
Imperial Oil Limited did this thing. However, Imperial failed to do this other thing.
You also do not need to formally define abbreviations that you don’t use again in the document (unless the abbreviation is better known than the full phrase, e.g., RCMP).