There are many subclasses on every class, whether they are user-made or item sets, they usually have some made up rules to make the game more fun/interesting, for example I'll link down below a medic subclass guide I found rather interesting.
https://steamproxy.net/sharedfiles/filedetails/?id=2996776754

Also known as One Thousand and One Demoknights

• Ali Baba's Wee Booties
  
• Splendid Screen
  
• Persian Persuader
  
• Sultan's Ceremonial

Once attacked/in battle with an enemy, you may not run away until one of you is dead, even if that means dying.

You cannot use the shield for escaping situations or mobility, you may only use it to attack the enemy, or get closer to them.

Also known as Expert's Ordnance

• Loch-n-Load
  
• Ullapool Caber
  
• Scotch Bonnet

You may only use this subclass if you are playing on defense or in a mode where both teams can capture the point.

If someone from the enemy team is doing their objective you must stop doing what you just were and focus on attacking the players doing it, you may still attack the other enemy if you were in battle, but you must keep going to the capture point.

Also known as Swashbuckler's Swag

• Tide Turner
  
• Scottish Handshake
  
• Bootlegger
  
• Buccaneer's Bicorne
  
• A Whiff of the Old Brimstone

If there is water nearby, the player must immediately jump in it after using the shield, this does not apply if the nearest water is a death hazard.

After a medic heals them they must taunt to drink their beverage, even if it's mid-combat.

Also know as Highland Howl

• Ham Shank
  
• Hair of the Dog
  
• Scottish Snarl
  
• Pickled Paws

The player may only attack in dark areas, or on the entire map if the map is set on night time.

After every kill, the demoman must do the Wolf Howl taunt.

Also known as South of the Border Pack

• B.A.S.E. Jumper
  
• Bottle
  
• Allbrero
  
• Seeing Double
  
• Six Pack Abs

If the player wants to attack someone, they must first use the base jumper before doing so, no matter how little the height that they jump from is. This resets upon killing someone.

If the player's health is below 50, they need to get back to their base and heal themselves, health kits do not count.