My further response:
It is a good method - part 'direct instruction' and part 'discovery learning'.
Honestly - if I put every bit of code on the chart, we'd all give up before we start!
There are many more letter/s-sound correspondences which could be added to an Alphabetic Code Chart - but the designer of such charts has to make a judgement call on what to include or not.
I've just designed some 'table top' Alphabetic Code Charts which we are making available both as pre-made 'hard copy' (card) charts and as pdf printable charts (coming soon).
On these latest charts I've added 'wh' as code for /h/ as in the word 'who'. I've also added 'u' as code for short /oo/ as in the word 'push'.
Sometimes, a letter/s-sound correspondence can be 'rare' but in a very common word. The examples above are of more rare code but in common words.
I often refer to the word 'giraffe'. In this word, the grapheme 'ffe' is code for the /f/ sound - but I haven't included it on my Alphabetic Code Charts.
This is such a rare grapheme in a very commonly 'known' word - but not necessarily a commonly 'used' word - whereas the words 'who' and 'push' have rare graphemes but they are commonly known
and commonly used.
Quite frankly - these are judgement calls - but by noting that this is the case, and by encouraging parents or teachers to 'add' to the Alphabetic Code Chart as new graphemes are discovered, then this is a happy solution which works very well!