Does Cast a Spell make you a spellcaster?

Question

If I want to use a Spellheart to cast Electric Arc as a Monk, I need the Cast a Spell ability. Based on the comments on this answer, people seem to think Focus Spells gives you Cast a Spell for Focus Spells only.

However, the the rule does not explicitly say this, it says you do not become a spellcaster.

However, you don’t qualify for feats and other rules that require you to be a spellcaster.

In the description of Cast a Spell, I could not find anything about being a Spellcaster.

Is requiring Cast a Spell the same as requiring you to be a Spellcaster? (And where is Spellcaster defined?)

Answer 1

Spell slots make the spellcaster.

Chapter 7: Spells implies throughout that being a spellcaster is closely related to a character's class, but gets to the point in the paragraph about Non-Spellcasters with Focus Spells:

If you get focus spells from a class or other source that doesn’t grant spellcasting ability, the ability that gives you focus spells also provides your proficiency for your spell attack modifier and spell DC, as well as the magical tradition of your focus spells. Though you can cast your focus spells, you don’t qualify for feats and other rules that require you to be a spellcaster or have a spellcasting class feature—those require you to have spell slots.

A similar sentence is repeated in the section about Innate Spells.

Essentially, you need spell slots to be a spellcaster. Normally, a character gets spell slots from their spellcasting class feature, but a character with spell slots from a different source (such as a spellcasting archetype's Basic Spellcasting feat) still counts as spellcaster that has the spellcasting class feature.

And some rules do indeed require a spellcasting class feature, like the Activate an Item activity:

If an item lists “Cast a Spell” after “Activate,” the activation requires you to use the Cast a Spell activity to Activate the Item. This happens when the item replicates a spell. You must have a spellcasting class feature to Activate an Item with this activation component. If the item can be used for a specific spell, the action icon for that spell is provided. If it's an item like a staff, which can be used for many spells, the icon is omitted, and you must refer to each spell to determine which actions you must spend to Activate the Item to cast it.

That is at the crux of the Q&A that prompted your question: a monk, even one with Qi Spells, can't activate a spellheart because they don't have a spellcasting class feature. To do so, a monk would need a feat like Basic Druid Spellcasting to obtain spell slots, or a feat like Trick Magic Item to skirt around the requirement.

Answer 2

Focus Spells do not make you a Spellcaster

To preface, there are two different actions called Cast a Spell:

• Cast a Spell from spellcasting, and
• The Cast a Spell item activation (this one is the one used for Spellhearts)

From the Cast a Spell item activation:

You must have a spellcasting class feature to Activate an Item with this activation component.

Focus spells do not give the Spellcasting class feature. From Non-Spellcasters with Focus Spells:

you don’t qualify for feats and other rules that require you to be a spellcaster.

Caster archetype dedications also don't give you the spellcasting class feature. For archetypes, you need the Basic Spellcasting feat to activate Spellhearts. From Spellcasting Archetypes:

A spellcasting archetype allows you to use scrolls, staves, and wands in the same way that a member of a spellcasting class can, and the basic spellcasting feat counts as having a spellcasting class feature.