Use the vocabulary of your reader. That's a key factor in plain language.
- If you are writing for children, use words they understand.
- If you are writing for doctors, use correct medical terms.
- If you are writing for the general public, use the same kind of words they would use.
Remember what your high school English teacher told you? She gave you credit for introducing surprising, unusual vocabulary. That was evidence of cognitive learning, and mental agility.
Now forget it! That was appropriate advice for writing high school book reviews, academic articles and 'creative writing' such as literary fiction. In those cases your readers appreciate evidence that your brain is working hard, and are willing to stretch their own brains.
But when you're at work, you are writing for someone who needs information: clear information in a hurry.
The success of your communication depends on one thing: does the intended reader understand it? If not, you've failed. In the big bad world of business, you don't get extra points for mysterious language—you get points deducted.
Fancy words from an airline trying to impress
This functional bag contains specially selected products chosen for their ability to enhance your wellbeing.
Same message in words most passengers will understand
This bag contains stuff that may make you feel better.
(Clarity becomes embarrassing when the message is pointless.)