Special Ability: Point Defense (X)
This is a variety of light, short-range weapons designed as a close-in anti-infantry defense. This can be a
combination of reactive armor, energized hulls and automated weapons systems, though there are different
mechanisms, i.e. Ork Point Defense generally consists of Gretchen with pistols, tied to the hull.
Point Defense systems (the X listed next to the ability) always act as though they were on First Fire Orders
no matter what the unit’s actual orders are, and may be thought of as a separate unit. They may be used in the
Movement Phase in order to Snap Fire at the target of a charge (with the normal –1 To-Hit penalty) or to gun down a
Charging enemy (without the –1 penalty), or they may fire in the Combat Phase when the model make it’s normal
ranged attacks.
All PD attacks have a 360° arc of fire, a range of 15 cm, hit on a 6+ and have 0 TSM. Point Defense

Can I use Snapfire with a unit that has been activted? Or is it not an activation?

Can my Landraider that is being charged use its PD, see if it works and then if not choose to show its FF order and fire its lasers?

Special Ability: Point Defense (X)
[...]and may be thought of as a separate unit[...]

and may be thought of as a separate unit.

So a unit can never use its PD if it has been moved/been activated?

Yes they can use their PD if they have moved/activated.

...because PD may be thought of as a separated unit from the land raider. If you have activated your land raider detachment, even if they are on charge order, you can snap fire with their PD because they (the PD) have not been activated.

To make it easier, with a land raider detachment, you have 2 units in one :
- The land raider wich can moove, fire, engage close-combat, etc...
- The PD wich are always in first-fire order and which can only fire.