Suitability and sustainability are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It suggests what word you may want to save time, but quite often, its suggestions couldn’t be more off base and produces humorous results.
Suitability is an adjective. It describes something that fits, is appropriate, or complements a design or message.
Sustainability is an adjective. It means an industry or practice that can survive for the long term because it doesn’t grow beyond what can be supported, and it doesn’t exhaust resources that are needed for its existence.
The following story uses both words correctly:
Susie had just graduated college with a degree in economics and was interviewing with many firms. She considered the suitability of each firm with her career goals, and whether those companies practiced sustainability.