The 3D view always has a currently selected cursor plane, selected from the 'View' tab of the taskbar.
The cursor plane selection defines the way that 2D movement of the mouse cursor is interpreted into 3D movement in the 3D view. The 3D movement must occur in some 3D plane. If vertically orientated objects are under construction one of the vertical cursor planes should be used (i.e. the XZ or YZ plane).
Sometimes cursor movement is restricted, perhaps locked to a center line, object node, cartesian axis or other. In these instances the cursor movement is more strictly controlled in the view although the cursor plane setting may have some bearing on the 3D interpretation of the mouse position. For example in the case of 'Axis Snapping ' when rubber banding the cursor is restricted to alignment with the nearest cartesian axis (X, Y or Z) but the nearest axis will inevitably be one of the cursor plane free axes.
Generally during 3D point selection one of the axes is static (or locked) depending on the cursor selection (i.e. the Z axis value is static when the XY cursor plane is selected). The static value can usually be changed by directly inputting values into the static axis edit box.