The material below is Open Game Content from a 3PP Source

Combat Modifiers

Attacker is…	Melee	Ranged
Dazzled	–1	–1
Entangled	–21	–21
Flanking defender	+2	—
Invisible	+22	+22
On higher ground	+1	+0
Prone	–4	—3
Shaken or frightened	–2	–2
Squeezing through a space	–4	–4

¹ An entangled character also takes a –4 penalty to Dexterity, which may affect his attack roll.
² The defender loses any Dexterity bonus to AC.
³ Most ranged weapons can't be used while the attacker is prone, but you can use a crossbow or shuriken while prone at no penalty.

Defender is…	Melee	Ranged
Behind cover	+4	+4
Blinded	–21	–21
Concealed or invisible	See Concealment
Cowering	–21	–21
Entangled	+02	+02
Flat-footed	+01	+01
Grappling (but attacker is not)	+0	+0
Helpless	–43	+03
Kneeling or sitting	–2	+2
Pinned	–43	+03
Prone	–4	+4
Squeezing through a space	–4	–4
Stunned	–21	–21

¹ The defender loses any Dexterity bonus to AC.
² An entangled character takes a –4 penalty to Dexterity.
³ The defender is denied his Dexterity bonus to his Armor Class.

A number of factors and conditions can influence an attack roll. Many of these situations grant a bonus or penalty on attack rolls or to a defender's Armor Class.