Two kinds of tab-like action are provided in Picture mode.
Context-based tabbing is done with M-<TAB>
(picture-tab-search). With no argument, it moves to a point
underneath the next “interesting” character that follows whitespace in
the previous non-blank line. “Next” here means “appearing at a
horizontal position greater than the one point starts out at”. With an
argument, as in C-u M-<TAB>, the command moves to the next such
interesting character in the current line. M-<TAB> does not
change the text; it only moves point. “Interesting” characters are
defined by the variable picture-tab-chars, which contains a string
of characters considered interesting. Its default value is
"!-~".
<TAB> itself runs picture-tab, which operates based on the
current tab stop settings; it is the Picture mode equivalent of
tab-to-tab-stop. Without arguments it just moves point, but with
a numeric argument it clears the text that it moves over.
The context-based and tab-stop-based forms of tabbing are brought
together by the command C-c <TAB> (picture-set-tab-stops.)
This command sets the tab stops to the positions which M-<TAB>
would consider significant in the current line. If you use this command
with <TAB>, you can get the effect of context-based tabbing. But
M-<TAB> is more convenient in the cases where it is sufficient.