When can you "resist" a dimensional lock?

Question

The spell Dimensional lock has the following description:

You create a shimmering emerald barrier that completely blocks extradimensional travel. Forms of movement barred include astral projection, blink, dimension door, ethereal jaunt, etherealness, gate, maze, plane shift, shadow walk, teleport, and similar spell-like abilities. Once dimensional lock is in place, extradimensional travel into or out of the area is not possible.

A dimensional lock does not interfere with the movement of creatures already in ethereal or astral form when the spell is cast, nor does it block extradimensional perception or attack forms. Also, the spell does not prevent summoned creatures from disappearing at the end of a summoning spell.

It also says:

Spell Resistance yes

When are you supposed to check for spell resistance?

• When you are trying to "force" the lock by using an extradimensional way of travel?

• When the designated "point in space" is positioned on a creature?

  Area 20-ft.-radius emanation centered on a point in space

• Some other time?

Answer 1

You check Spell Resistance once, when a creature enters the spells area.

You check spell resistance of any spell individually against any creature that has it when they enter the area of the spell for the first time. For example, if a devil of some sort is in the area of Dimension Lock when you cast the spell you make the check against it immediately. If a demon walks into the spells area several days later, you make the check at that point. If a creature leaves the spells area and returns, it keeps the previous success or failure that you rolled, and you do not re-check against SR. As a side note, a creature will not necessarily know whether you have or haven't overcome its SR until it tries to teleport and either succeeds or fails.

From the SRD page entry for Spell Resistance

Check spell resistance only once for any particular casting of a spell or use of a spell-like ability. If spell resistance fails the first time, it fails each time the creature encounters that same casting of the spell. Likewise, if the spell resistance succeeds the first time, it always succeeds. If the creature has voluntarily lowered its spell resistance and is then subjected to a spell, the creature still has a single chance to resist that spell later, when its spell resistance is back up.

Further, in regards to area spells specifically, it has the following to say:

Spell resistance applies if the resistant creature is within the spell’s area. It protects the resistant creature without affecting the spell itself.

Also, an important note on targeting the spell:

You can't attach the spell to a creature. This spell specifically restricts it to a point in space and does not allow it to be attached to a creature or object. Spells that allow this will say so, like Silence, which specifically says "a creature, an object, or a point in space".