an ambiguous move is one that does not fully indicate the piece to be moved and its destination, such as "I move my bishop back", "castling", or "I'm gonna queen a pawn".

No. You obviously haven't reached "Recording of the Moves" and clarifications given in the glossary. A move, to be a move, has to be written in algebraic notation. None of the expressions you have written qualify as moves let alone ambiguous moves.

If the player had rooks on a7 and c7 and he wrote Rb7 then that would be ambiguous.

What is the meaning of clause b. above?

If the person trying to read the move can't work out what the move is because part or all of it is illegible then that would qualify. In general this a catch-all clause that basically says you can't work out what the move is for whatever reason.

an ambiguous move is one that does not fully indicate the piece to be moved and its destination, such as "I move my bishop back", "castling", or "I'm gonna queen a pawn".

No. You obviously haven't reached "Recording of the Moves" and clarifications given in the glossary. A move, to be a move, has to be written in algebraic notation. None of the expressions you have written qualify as moves let alone ambiguous moves.

If the player had rooks on a7 and c7 and he wrote Rb7 then that would be ambiguous.

What is the meaning of clause b. above?

If the person trying to read the move can't work out what the move is because part or all of it is illegible then that would qualify. In general this a catch-all clause that basically says you can't work out what the move is for some reason other than ambiguity. Complete absence would also do. i.e. the player seals a blank sheet of paper.