net.ReadEntity returns nil, because entity is not in player's PVS? How to avoid?

  1. What is PVS?
  2. How can I make it so that it doesn’t return nil/another way to identify the entity client side even if the entity is fullway across the map?

So I researched a bit, and I still don’t know the what that full form if PVS(Plain View Sight?) but I found this:

function ENT:UpdateTransmitState()
     return TRANSMIT_ALWAYS;

I was wondering, when you use “SetNetworked*” functions, does the entity’s transmit state change to TRANSMIT_ALWAYS or is there some other way? Can I transmit the entity, let the client get an entity using net.ReadEntity and then untransmit the entity and still let the client work with the entity?

Is it safe to make many entities “TRANSMIT_ALWAYS”?

Also, does EntIndex change or are does every new ent get a new index?