null bits: One bit per column that can be NULL. The "null bits" are rounded up to the next byte boundary. the number of "null bytes" are also referred as data_offset.
fields: One field per column. No separators, length fields, etc. Length (pack_length) and layout of the fields depend on the field type:
-- to be added --
Note: The "in-memory record layout" is used by the SQL layer. It is independent from storage engines. All storage engines have to accept and produce this in-memory format at the handler interface.