Code 10/21 is specifically directed towards someone who has done something for you. Code 10/20 congratulates someone on a performance which may not benefit you in any way.
Code 10/21 is an acknowledgement of helpful and considerate behaviour. There is a very personal element in this code. It is not just a general feeling of being pleased and happy. It is a specific appreciation of some kindness. This kindness may have been outside the line of duty but may also have been within the line of duty. A patient may thank a surgeon for performing an operation even though this is his or her job.

It might be said that thanking someone is not exactly a 'mood'. It is, nevertheless, an expression of feeling. It is a short mood.