Applied loads, displacements and reactions are positive when they act along or about the positive axis direction. For rotations (like moments) the positive direction is with respect to the RIGHT HAND RULE:
Be wary of the axes which are used to define loads. By default, loads are along the global axes however distributed loads can be defined with respect to the local member axis. Similarly, plate pressures can be defined globally or locally to the plate. Also remember that the local axes of members and plates can be viewed and toggled from the Visibility Settings.
In the diagram above:
The diagram above can be summarized as follows:
NOTE: There is a setting in Settings called "Reverse BMD Sign" which reverses the sign convention of the bending moment diagrams to suit whichever convention you're most familiar with.
By default, a simple downward distributed load with two point loads will display the following bending moment diagram:
If the "Reverse BMD Sign" setting is toggled on, the exact same model would produce the following bending moment diagram: