A class and a subclass are related, but they are not interchangeable. The class is the character's main mechanical progression. The subclass is a specialization added to that progression.

What the class provides

The class normally determines things such as:

  • Hit Die;
  • starting proficiencies;
  • saving throw proficiencies;
  • weapon and armor access;
  • spellcasting progression, if any;
  • class resources;
  • major recurring features;
  • Ability Score Improvement or feat progression;
  • when a subclass is selected.

A 5th-level Rogue is still fundamentally using the Rogue class chassis regardless of whether the character is a Thief, Assassin, Arcane Trickster, Soulknife, or another Roguish Archetype.

What the subclass provides

A subclass adds a more specific identity and set of features.

It may change the character's preferred tactics, add new proficiencies, grant spells, introduce a resource, improve an existing class feature, or provide an entirely new secondary role.

The exact amount of mechanical change varies considerably between classes and subclasses.

Different classes choose subclasses at different levels

There is no universal subclass level across all Fifth Edition material.

In the 2014 rules, for example:

  • Clerics and Sorcerers choose their subclass at 1st level;
  • Druids and Wizards choose at 2nd level;
  • many martial and hybrid classes choose at 3rd level.

Additional classes can use their own structures.

Messenger chooses a Messenger Path within its own ten-level calling progression.

Shinobi chooses a Shinobi Path at 3rd level while also having a separate Chakra Origin system.

Pugilist chooses a Fight Club at 3rd level.

Subclass terminology is class-specific

The word “subclass” is a useful general category, but published rules often use a thematic name.

Examples in D&D Portal include:

Artificer → Artificer Specialist Barbarian → Primal Path Bard → Bard College Cleric → Divine Domain Druid → Druid Circle Fighter → Martial Archetype Monk → Monastic Tradition Paladin → Sacred Oath Ranger → Ranger Archetype Rogue → Roguish Archetype Sorcerer → Sorcerous Origin Warlock → Otherworldly Patron Wizard → Arcane Tradition Messenger → Messenger Path Shinobi → Shinobi Path Pugilist → Fight Club

The wiki uses each class's own specialization term rather than forcing every option to be called a “subclass.”

A subclass does not remove the base class

Choosing a subclass does not replace the previous class features.

The character continues gaining the normal class progression and gains subclass features at the levels specified by that class.

That matters when comparing builds.

A subclass may strongly influence tactics, but most of the character's rules still come from the base class.

Subclass versus multiclass

A subclass is not the same as multiclassing.

Choosing a subclass keeps the character inside one class progression.

Multiclassing means taking levels in more than one class.

For example:

Rogue 5 / Fighter 3

is a multiclass character.

A level 8 Rogue with the Swashbuckler Roguish Archetype is not multiclassed.

In short

The class is the foundation.

The subclass is the specialization.

Use the class-specific name for its specialization; different classes may call that choice a Path, Domain, Order, College, Tradition, Contract, Guild, or another source-specific term.