Recommended lower speed (E11)

"The sign indicates a stretch of road or an area where specific measures have been taken or where conditions are such that it is advisable to drive at a lower speed than the maximum permitted speed. The recommended lower speed is indicated on the sign."

Recommended lower speed (E11)

Speed limit (C31)

"The sign indicates that it is prohibited to drive a vehicle faster than the speed indicated on the sign."

Speed limit (C31)

Conclusion

E11 (square blue/white) = You may drive faster, but its not recommended. You cant drive faster then the roads normal speed limit C31 (round yellow/red/black) = Permitted speed of the road, You cant drive faster then the sign as its illegal to drive over the speed limit