Constants are names, recognized by TV as values. Constants are:
B = 0

Z = size of buffer 
    Integer value, denoting the current size of the buffer.

. = current position
    Integer value, denoting the offset (in characters) from the beginning
    of the buffer. A position of 0 would mean that the current position
    is at the beginning of the buffer. Similarly, a position of 100
    indicates that there are 100 characters between the start of the
    buffer and current position.

H = current buffer
    Range 0 , Z , denoting the entire current buffer. This is one of only
    two constants of type range.

N = current line
    The range denoting the entire current line. This is one of only two
    constants of type range. The RETURN character at the beginning
    and end is NOT included by default. If the N is immediately followed
    by a +, the returns at the beginning and end are included. This
    allows anchoring of a search-string to line-begin or end.
    NOTE 1: the first line has no leading return, and the last line may
    not have a trailing one.
    NOTE 2: A search on a line with a return may end on the next line.
    NOTE 3: This constant is special in yet another way: it accepts an
    argument, specifying how many lines to include in the selection in addition
    to the current line. A negative argument looks back instead of forward.

As is clear from the above, the term 'constants' is not appropriate for
everything listed here, as many of them are constant only for as long as
the buffer or even the cursor position does not change.