What does “using the instrument as the source of the spell” mean with instruments of the bards?

Question

Each bardic instrument ends with these sentences:

When you use the instrument to cast a spell that causes targets to become charmed on a failed save, the targets have disadvantage on the saving throw. This effect applies whether you are using the instrument as the source of the spell or as a spell casting focus.

Yet, it appears that only the Doss Lute (which can cast Animal Friendship) has a spell that causes targets to become charmed. None of the other instruments actually have such a spell.

For example:

• Canaith Mandolin: Fly, Invisibility, Levitate, Protection from Evil and Good, Cure Wounds (3rd level), Dispel Magic, Protection from Energy
• Cli Lyre: Fly, Invisibility, Levitate, Protection from Evil and Good, Stone Shape, Wall of Fire, Wind Wall
• Anstruth Harp: Fly, Invisibility, Levitate, Protection from Evil and Good, Control Weather, Cure Wounds (5th level), Wall of Thorns

How can the instrument be the source of a spell causing charm if it doesn't have spells that charm? Can the origin of spells cast by a bard can be shifted to items? Why is the phrase "...when using the instrument as the source of the spell..." used in the instrument descriptions and what does it mean?

Answer 1

The phrase means casting a spell from the item

This effect applies whether you are using the instrument as the source of the spell or as a spell casting focus.

Using an item as the source of a spell can really only mean using the item like a wand or rod that contains a spell. There is simply no other mechanical meaning it can have. No other rule or mechanic enables casting from the item in a way that does not involve using the item as an arcane focus or casting a spell that is contained within the item (using the item as a spell's source).

The Doss Lute can cast a spell that causes the charmed condition

You are not entirely correct when you say "none of them have spells that charm" though. The Doss Lute has the spell animal friendship which says:

...the beast must succeed on a Wisdom saving throw or be charmed by you for the spell's duration.

But that is indeed the only spell that causes the charmed condition out of all the instruments of the bards.

In fact, the only other spell period that has a charm effect and a material component is hypnotic pattern.¹

As to why the designers did it this way, we can only speculate. It seems likely that it was an oversight and that the items were intended to cast more charm spells at one point in development. Or it could just be future proofing for further instruments to be added later (potentially with more spells that cause the charmed condition).

^{1 - See What spells can an Instrument of the Bards actually impose disadvantage on? for more on the background of this issue.}

Answer 2

It appears to be a formulaic construction that they will use for all such instruments - some in the future may be able to cast spells that charm even if none of these do.

If the bard uses the instrument as a focus to cast a spell with a material component that imposes the charmed condition then the second limb kicks in. Right now, of all the spells that charm (see Fey Ancestry - What spells and effects are covered by 'advantage against being charmed'), only Hypnotic Pattern has a material component.