According to this figure whats the difference between leading edge flap and slat and why the slat come with plain wing and TE flap come alone? thanks.
This comes down to the fact that flaps are fundamentally changing the geometry of the wing so with flaps deployed the wing is a different shape with correspondingly different characteristics. So when a flap is deployed it is essentially a modification of the main wing.
On the other hand the primary role of slats (distinct form LE flaps) is to control the airflow over the wing, specifically to delay flow separation, and as such don't directly contribute lift themselves but allow the main wing to operate at a higher angle of attack before it stalls.
Some aerofoils also have slots which allow airflow between low and high pressure sides of the wing with the intention of either reducing drag or keeping the airflow attached or both. It's not uncommon to see flaps with slot gaps between the training edged of the main wing and the leading edge of the flap itself.