Note:The rules could be updated or changed in the future.
1.CATALYSTS:You must use a catalyst that holds meaning to or belongs to the 'Human' or 'Monster' that you summon. The catalyst in the summoning ritual will be destroyed upon the servants summoning, so it is recommended to have multiple catalyst for the servant should they die and you want to re-summon them.
2.COMMAND SPELLS:You will have 3 commands on your soul instead of on one of your arms. Command seals will allow you to make absolute orders on your servants, though depending on how powerful the servant is you might need to use more than 1 command seal. The command seals can also be used to power up servants and even use it to teleport them to you. To have your servants use their Ultimate abilities, you must use 3 command seals to unleash it. It takes 24 hours to regain 1 command seal.
3.NO COPY SERVANTS:You can not have two of the same servant unless they have Severe differences such as [From different AUs][Different history or Abilities][A younger version or future version][Demi-Servant][Alter Ego].
4.SERVANT RESTRICTION LIMIT AND EXCEPTIONS:There is a limit to the types of 'Human' or 'Monsters' that you can summon from the Underverse, but there are those that you can't summon such as [Gods][Full-Hate Entities][Servant that refuse to be summoned][Restricted Servants][Fully Corrupted Error Entities].The types of servants that you can most commonly summon are 'Humans' and 'Monsters', but there are some exceptions such as summoning 'Robots' like Mettaton, or 'Other worldly beings' that are neither 'human' nor 'monster'.
5.SERVANT KNOWLEDGE AND INFORMATION:The servant you will summon will have basic knowledge that a servant normally gets. The servant is pretty much a perfect copy from their original, though their memories will be limited with only basic knowledge. Depending on the servant you would get little to no information about them unless they tell you or if you start having a bond with them, you will gain their memories through your dreams.
6.COMMAND CODES:You can create your own command codes by crafting them out of catalysts. You can only equip 5 command codes to 1 servant each.
7.SUMMONING CONDITIONS: Some servants or specific classes will have specific summoning conditions to fulfill in order to summon them. For example if you want to summon an Avenger class servant, you must not only use a catalyst, but use a lot of you're blood in the summoning.
8.CATALYST DETECTION: You can see and detect catalysts near you.