Hi, i just finished waterproofing with kerdi and the tiles i have are 12x12 porcelain and the box they came in says to use a thinset with chemiccal additives which have elasticity characteristics. Im not sure what that means, but with kerdi they recomend unmodified. any sugestions.. Thanks..Clint

Hi Clint
Yes, unmodified is required although modified may be used if you are able to wait a week or so before you grout.
Modified mortar needs air to hasten the curing process whereas unmodified does not.

I have had to repair a porcelain tile which was set into unmodified mortar over Ditra after only 2 days after original installation. The tile had to be broken into a million pieces in order to be removed .... it was well bonded.
For wall installations I always back-butter my larger tiles to obtain the best possible bond over Kerdi.

Thanks Harry, i went with a modified (ultraflex 2) hope that was ok, should i wait a week before grouting?

Yes ... wait at least a week.
The larger the tile ... the more important it is to enable the mortar to cure.