I'm not certain offhand where this is documented, but whenever you see ___ in the Syntax section of a function reference page it indicates that you can use any of the other syntaxes for that function in place of the ___ and then add on whatever additional input or output arguments are listed in that particular syntax. So based on the first four lines of the Syntax section:
quiver(X,Y,U,V)
quiver(U,V)
quiver(___,scale)
quiver(___,LineSpec)
you can call quiver as:
quiver(X, Y, U, V) % Just Syntax 1
quiver(U, V) % Just Syntax 2
quiver(X, Y, U, V, scale) % Filling in the blank in Syntax 3 with Syntax 1
quiver(U, V, scale) % Filling in the blank in Syntax 3 with Syntax 2
This avoids "combinatorial explosion" -- we don't have to say that both these syntaxes are allowed. For the case where you have just two syntax entries with ___ you don't save any space, but the Syntax section of quiver has six syntax entries with ___ -- listing all the combinations explicitly would be very long.
% Fill in Syntax 3's blank with Syntax 1, then put the result in Syntax 4's blank
quiver(X, Y, U, V, scale, LineSpec)
% Fill in Syntax 4's blank with Syntax 1, then put the result in Syntax 3's blank
quiver(X, Y, U, V, LineSpec, scale)
