Circuits, as published and used by Elektor and the Dutch Elektuur, contain universal transistors and diodes to the abbreviations: TUP (Transistor Universal Pnp), TUN (Transistor Universal Npn), DUS (Diode Universal Silicon), and DUG (Diode Universal Germanium). Many transistors and diodes fit this way in these categories and makes component selection easier. Good system!

TUN/TUP
The minimum specifications have to be met, 
in Table 1a above, 
before you can call it a 
'TUP' or a 'TUN'.
DUS/DUG
The minimum specifications have to be met, in
Table 1b above, 
before you can call it a 'DUS' 
or a 'DUG'.
TUN/TUP Transistors
In the above tables, Table 2 & 3, you 
can use several different transistor types for a 
TUP or a TUN. 
 Obviously the tables are not 
complete.  It would be almost impossible to list 
all available transistor types available today.
  
From the above listed types are all A, B, or C 
types usable.
DUS/DUG Diodes
Several different types of diodes are suitable
as a 'DUS' or 'DUG'.
Table 1a - TUN/TUP2
The most important parameters of the BC107...BC109 
and the BC177...BC179. 
 These transistors have been 
chosen as an example of information.
Case Outlines
The letter after the transistor indicates the hfe.  
Example: BC107A, hfe = 125 ... 260
         BC107B, hfe = 240 ... 500
         BC107C, hfe = 450 ... 900
Substitutes within the BC series of transistors are also possible. In Table 6 you see that the transistors are grouped in three. Example, the BC107, BC147, BC317 and BC413 can be substituted with each other, but a BC548 may not be exchanged for a BC107. Why? The BC548 is the second of a group of three. Your choice would be a BC547(A,B, or C).