At the same time, anyone can understand my model

I don't see how that's relevant since the table you posted does not come close to representing encoding.

Both the input side and the output side of your table is decoded, so it represents neither encoding nor decoding.

Encoding = convert unicode integer to utf-8 character for output

I've never heard of "unicode integer" before. Neither has Google. Most people say "unicode character" or "unicode code point".

"UTF-8 characters" is commonly used to mean for both "the character encoded using UTF-8" and "the bytes resulting from encoding a character using UTF-8". The former is the result of decoding, the latter is the result of encoding. As such, it's meaningless/confusing/unclear how you used it.

That simple model will allow any unicode beginner

I'm an expert and I have no idea what you mean by those two lines.

Fixed terminology:

• Encoding = Converting unicode characters (e.g. black spade suit) into utf-8 bytes (E2 99 A0) for storage or transmission.
• Decoding = Converting utf-8 bytes (e.g. E2 99 A0) into the unicode characters (black spade suit) they represent in order to do string manipulations such as counting and comparing characters.

And now for something short and clear:

• Encoding is necessary to store text into a file since files can only contain bytes.
• Decoding gives you back the text you stored into the file.