How are Ties different from Slurs

[Urs Liska]

Hi list,

I tried to adapt the shapeSlur_rev.ly function recently posted by David 
Nalesik to also work for PhrasingSlurs and Ties.

However, when I use

  \once \override Tie #'control-points = #(shape-slur $offsets)

as part of the function, the Tie isn't printed at all.
Is there any difference between Slurs and Ties that I should take care 
about?

Sorry if this could be also found in the manuals, but I'm still too 
confused about all this to be able to find things straightforward.

Thanks
Urs