https://player.vimeo.com/video/750284521?h=49f16166db&badge=0&autopause=0&player_id=0&app_id=58479
Enabled by default, the direction for the particles is shown by an arrow in the viewport.
nxDirection arrow when Visible in Editor is enabled.
By default, this is set to Use Modifier Rotation.
The other options are: Relative, Absolute, Circular, Jitter and Ring.
With this mode, the direction that the particles will take is the direction in which the modifier is pointing.
This lets you simply rotate the modifier to point in the desired direction, without having to enter a heading and pitch value.