The custom difficulty settings are SO COOL. Sometimes my reaction times are slowed when I’m hit with fatigue or co-ordination based impairments.

Sometimes simply changing the difficulty to an overall easier one isn’t very fun and is usually *too* easy for me, so being able to say, make the enemy reaction times slower but my visibility higher keeps it challenging but gives me room to react (:
Cherryrae, via Twitter

Opening up as many existing variables as possible to player configuration can allow the same basic mechanic to be opened up to a huge range of abilities.Through settings the same base game can become everything from a simple cause and effect activity all the way through to a complex highly motor-demanding game.

It also allows gameplay to be tailored to the unique needs of each individual player, rather than being funnelled into simplistic general “difficulty” buckets which may not match up with the areas of gameplay that each player finds a particular barrier and which they do not.

The more settings exposed the greater the degree of personalisation possible, although care must be taken to avoid an overwhelming array of options. Presets and grouping can help greatly with this.

Best practice example: Way of the Passive Fist
Best practice example: Celeste
Best practice example: Dishonored 2
Best practice example: Even The Ocean
Best practice example: FIFA 14
Best practice example: SYNSO:CE
Best practice example: Something Special: Out and About

More information: Designing Universally Accessible Games PDF