What's the use case of KDE_INSTALL_DATADIR vs DATA_INSTALL_DIR

[Albert Astals Cid]

Hi, can somebody please help me understand the need of KDE_INSTALL_DATADIR vs 
DATA_INSTALL_DIR?

http://quickgit.kde.org/?p=blinken.git&a=commit&h=3121f70a7d755b3711c90a4f85f951e23725930d

http://quickgit.kde.org/?p=blinken.git&a=blobdiff&h=03631e2183bd4f67f398b17eb76b0e59dd35390b&hp=362f4e1efe2d3554ffc8bc2f1176d653596d6924&hb=3121f70a7d755b3711c90a4f85f951e23725930d&f=CMakeLists.txt

Thanks :)

Cheers,
  Albert