|Used for to-do notes, consider using fixme key instead|
|Used on these elements|
|Tools for this tag|
The key todo=* is used to make to-do notes in the map data. For example, open ends of ways may be tagged with todo=continue, meaning the way can still be continued and it's just not mapped yet (see this tag on taginfo).
Nowadays, the key fixme=* is far more popular than todo=*. Also, fixme=* is supported by many tools such as Keep Right, JOSM, Vespucci, iD, Osmose and many other. todo=* has minimal support, usage etc.
Therefore it is recommended to use fixme=* instead.
Current state of TODO=*: