Heightened Spontaneous Spells
Source Player Core pg. 296
If you're a spontaneous spellcaster, you must know a spell at the specific rank that you want to cast it in order to heighten it. You can add a spell to your spell repertoire at more than a single rank so that you have more options when casting it. For example, if you added skyfire wings to your repertoire as a 3rd-rank spell and again as a 7th-rank spell, you could cast it as a 3rd-rank or a 7th-rank spell; however, you couldn't cast it as a 5th-rank spell.
Many spontaneous spellcasting classes provide abilities like the signature spells class feature, which allows you to cast a limited number of spells as heightened versions even if you know the spell at only a single rank.
As a spontaneous caster, you can also choose to cast a lower-rank spell using a higher-rank spell slot without heightening it or knowing it at a higher rank. This casts the spell at the rank you know the spell, not the rank of the higher slot. The spell doesn't have any heightened effects, so it's usually not a very efficient use of your magic outside of highly specific circumstances. For instance, if your party was having trouble with an invisible enemy, and you had revealing light in your repertoire but had already spent all of your 2nd-rank spell slots, it might be worth it to use a 3rd-rank spell slot to cast the spell, even though it'd have no heightened benefit.
Many spontaneous spellcasting classes provide abilities like the signature spells class feature, which allows you to cast a limited number of spells as heightened versions even if you know the spell at only a single rank.
As a spontaneous caster, you can also choose to cast a lower-rank spell using a higher-rank spell slot without heightening it or knowing it at a higher rank. This casts the spell at the rank you know the spell, not the rank of the higher slot. The spell doesn't have any heightened effects, so it's usually not a very efficient use of your magic outside of highly specific circumstances. For instance, if your party was having trouble with an invisible enemy, and you had revealing light in your repertoire but had already spent all of your 2nd-rank spell slots, it might be worth it to use a 3rd-rank spell slot to cast the spell, even though it'd have no heightened benefit.