Whats the difference on .ReturnValue and .Output?

I use a Firebird 2.5 an call a stored procedures.

The called server stored procedure
CREATE OR ALTER PROCEDURE P_BLOCK_KONTROLLE (
aufruftitel varchar_255,
testext_in varchar_255)
returns (
blockid id,
istzeit zeitstempel,
rueckgabetitel varchar_255,
testext_aus varchar_255)
as
begin
blockid = 10;
istzeit = current_timestamp;
rueckgabetitel = aufruftitel;
testext_aus = testext_in;
suspend;
end


The vb-code, that calls the procedure
Sub AufrufStoredProcedure()

Dim cn As New FbConnection(strDBConn)
cn.Close()
cn.Open()
Dim cmd As New FbCommand("p_block_kontrolle", cn)
cmd.CommandType = CommandType.StoredProcedure

Dim fbParamI_1 As FbParameter = cmd.Parameters.AddWithValue("A", "Hello World")
Dim fbParamI_2 As FbParameter = cmd.Parameters.AddWithValue("E", "Simple test text")

Dim fbParamR_1 As FbParameter = cmd.Parameters.Add("B", FbDbType.Integer)
Dim fbParamR_2 As FbParameter = cmd.Parameters.Add("C", FbDbType.TimeStamp)
Dim fbParamR_3 As FbParameter = cmd.Parameters.Add("D", FbDbType.VarChar, 255)

fbParamI_1.Direction = ParameterDirection.Input
fbParamI_2.Direction = ParameterDirection.Input



' declaration with ReturnValue
fbParamR_1.Direction = ParameterDirection.ReturnValue
fbParamR_2.Direction = ParameterDirection.ReturnValue
fbParamR_3.Direction = ParameterDirection.ReturnValue

cmd.ExecuteNonQuery()
Debug.Print("Version with ReturnValue")
Debug.Print("Input-Parameter 1 = " + fbParamI_1.Value.ToString)
Debug.Print("Return-Parameter 1 = " + fbParamR_1.Value.ToString)
Debug.Print("Return-Parameter 2 = " + fbParamR_2.Value.ToString)
Debug.Print("Return-Parameter 3 = " + fbParamR_3.Value)


' declaration with Output
fbParamR_1.Direction = ParameterDirection.Output
fbParamR_2.Direction = ParameterDirection.Output
fbParamR_3.Direction = ParameterDirection.Output

cmd.ExecuteNonQuery()
Debug.Print("Version with Output")
Debug.Print("InputOutput-Parameter 1 = " + fbParamI_1.Value.ToString)
Debug.Print("Return-Parameter 1 = " + fbParamR_1.Value.ToString)
Debug.Print("Return-Parameter 2 = " + fbParamR_2.Value.ToString)
Debug.Print("Return-Parameter 3 = " + fbParamR_3.Value)

cn.Close()
cn.Dispose()
End Sub

With both declaration i get the requested result.
I do not see any difference.
Whats the reason for this two options?
Thanks for a short explanation.

I am just guessing (in an educated way) but I think this is a question for a VB forum.
There is just one set of out parameters as far as Firebird is concerned.
How VB treats/names them I do not know.

The reason to post this question on a Firebird Forum is, that the declarations made in the VB code all base on the
FirebirdSql.Data.FirebirdClient.dll (FBCommand, FBParamters and so on).
So the cmd-Variable is instance of FBCommand and fbparam is a instance/child of cmd.

fbparam has two characteristics (ReturnValue; Output).
Both are working fine, both are directly depending on the .DLL-Interface from Firebird. I just do not see a difference between. If they are equal I'm happy.