After reading up on processing algorithms I see there is not an option to have a "selected features" checkbox. It states in the documentation:
Algorithms follow the following simple rules when using a vector layer.
If the layer has a selection, only selected features are used. If there is no selection, all features are used.
However, when I run a process such as 'Random selection' from the Processing Toolbox all items (in my case all points), are processed, not just the features I have selected. I have gone into the Processing - Options - General menu and have selected/deselected 'Use only selected features' but no success?
Am I not proceeding correctly or is/was there a bug in v2.18?