dnd-5espells
Some spells, such as Forbiddance, say that under certain circumstances, they last "Until Dispelled". Other spells, such as Mordenkainen's Private Sanctum, say that under certain circumstances, they become "Permanent".
What is the difference between these two terms? Does "Permanent" mean that the effect can never be dispelled?
You create a ward against magical travel that protects up to 40,000 square feet of floor space to a height of 30 feet above the floor. For the Duration, creatures can't Teleport into the area or use portals, such as those created by the gate spell, to enter the area. The spell proofs the area against Planar Travel, and therefore prevents creatures from accessing the area by way of the Astral Plane, Ethereal Plane, Feywild, Shadowfell, or the Plane Shift spell.
Spell description doesn't specify the figure of the warded area, so we can assume we can shape it as we want. Red squares are (your teleportation circle area, 5x5x10 area) excluded from warded zone. The only way enemies can teleport in is to know the exact location of teleportation circle within the fortress. You can ward you fortress as following:
The text says "Planar travel is blocked" with no other exceptions or restrictions, which means that both of your specified scenarios would be blocked, regardless of how or why it is happening. Therefore, as long as the summoning spell is summoning a creature from another plane, it would be blocked.
For example, Instant Summons would be blocked if it summoned the object from another plane, and most of the Conjure X spells would be blocked as they pull creatures from another plane.
Best Answer
No difference.
Despite being an old question, I'll have to put here the other perspective. According to JC,
And according to Sage Advice,
Some spells do say "until dispelled", like Forbidance or True Polymorph, while others use the "permanent" nomenclature, like Private Sanctum. The discrepancy is strange, but it may have been an oversight from the designers. I think the idea is that, any magical effect created by a spell can be dispelled, thus falling in line with Dispel Magic's text.
There are no assumptions regarding the duration of the effect, only that its spell (and thus effect) is terminated. Also consider spells like Wall of Stone, that claim
So, despite being permanent, they specify how they can't be dispelled. This seems another indication that permanent just means "infinite duration".
As a DM, maybe you can house-rule it so that it is much harder (or even impossible) to dispel permanent effects (similar to Artifact magical items), but at least according to the designers, it seems that, as you asked, there is no difference between "until dispelled" and "permanent".