You can reach this mode...
with [y] onto the position the symbol handle shall appear
with "Compose Symbol" in Menu Components
with the tool under the icon
see also: Edit component
What you can do...
A component in schematic always consists of one or more symbols e.g. gates or OpAmps. A symbol itself is made of drawn elements (lines, arcs, pins etc.). Later these elements must be concatenated to a symbol. This works that way that you first select your self-drawn symbol (you may either modify an already existing component), eg one gate, hover the mouse to where the handle cross shall appear and then start this function. You can press [o] now for symbol options.
If you made a mistake and caught wrong elements together as a symbol, repeat this function highlighting (catching) the correct elements. If a handle was selected, this will be replaced by a new one.
If you want to export the whole component now press [x]. For modification of an already existing component, all of its symbols have to be inserted, reconcatenated with [y] and then exported again [x]. The "Edit Component" function of the Library Browser prepares every symbol this way, so that it can be reexported directly.
Leave this mode...
with [ESC] or M12