If a component is tagged as "RequiredComponentType" then the ship won't be available until all required components are available and fit. The difference to "ComponentType" is that a ship can miss a "ComponentType" but it can't miss a "RequiredComponentType". This affects when the ship appears as it depends on your tech research and miniaturization level.

The FillerComponentTypes cycle and skip unavailable components. Just to be sure it does so I tested the following stripped down blueprint:

  <ShipBlueprint>
    <InternalName>PSY_DestroyerBeamBP</InternalName>
    <ShipHullType>Small</ShipHullType>
    <Role>Capital</Role>
    <RequiredComponentType>BeamWeapon</RequiredComponentType>
    <ComponentType>BeamWeapon</ComponentType>
    <FillerComponentType>BeamWeapon</FillerComponentType>
    <FillerComponentType>Shields</FillerComponentType>
    <AllRequiredMustFit>true</AllRequiredMustFit>
  </ShipBlueprint>

Researched OrbitalManufacturing to get small hulls. The design didn't appear because it has RequiredComponentType=BeamWeapon. Researched WeaponsSystems. The ship appeared with 4 lasers meaning it had done:

RequiredComponentType: added 1 laser

ComponentType: added 1 laser

FillerComponentType: added 1 laser

FillerComponentType: skipped Shields (because no Defense tech)

FillerComponentType: added 1 laser

--> 4 lasers

Also, I forgot about the tag <AllRequiredMustFit>. The RequiredComponentType tag says that you must have the tech that allows the component for the design to appear but the component can be omitted if there's not enough space. If you want the ship to not appear at all if there's not enough space for all required components then you must also set AllRequiredMustFit=true. I have it true for all my designs so didn't even think about it...