Motorola MC68HC05T16 Technical Data Manual page 6

Register and bit mnemonics are defined in the paragraphs describing them.
An overbar is used to designate an active-low signal, eg: RESET.
Unless otherwise stated, blank cells in a register diagram indicate that the bit is
either unused or reserved; shaded cells indicate that the bit is not described in the
following paragraphs; 'u' is used to indicate an undefined state (on reset).
Conventions
TPG