Buffy l------------D-----------------------------------------D...l----------------
Kendra.................l-------------D
Faith.......................................l-----------------------------------------------
The above is a very crude way of showing the 'Slayer Line'.
Buffy is activated at "l" so she is the Slayer. At point 'D' Buffy 'Dies' so another Slayer is Called.
HOWEVER, because Xander 'revives' her, she doesn't completely die, though the 'Slayer Line' has already passed onto its next charge, Kendra.
With Buffy still alive, she has retained the abilities of the Slayer but lost the 'Slayer Line'. This now resides in Kendra.
When Kendra dies, the Line is then passed onto the next Charge, Faith.
Buffy is still alive here, but not the Slayer in the sense she doesn't hold the Line, BUT she is still a Vampire Slayer because thats what she does with her powers.
When Buffy dies again though, she can't pass the Line on because she doesn't have it.
Now, this is where things get a little odd.
When Buffy is brought back from a true death, somehow Willow puts her back in the 'Slayer Line'which creates confusion as now there really is 2 active Slayers, Faith and Buffy.
I hope this helps you Tiramisu or whoever you are..if not, someone can explain it better than me😉