\definefontfeature	[kacst]
			[mode=node,language=dflt,script=arab,
			 init=yes,medi=yes,fina=yes,isol=yes,
			 liga=yes,mark=yes,mkmk=yes]

\definefontfeature	[kacst-kern][kacst][kern=yes]
\font\nokern=KacstOne.ttf*kacst at 80pt
\font\yeskern=KacstOne.ttf*kacst-kern at 80pt

\starttext
\startTEXpage
\nokern\textdir TRT
صرصرصرصر
\stopTEXpage
\startTEXpage
\yeskern\textdir TRT
صرصرصرصر
\stopTEXpage
\startTEXpage
\yeskern
صرصرصرصر
\stopTEXpage
%\showotfcomposition{KacstOne.ttf*nokern}{-1}{صرصرصرصر}
%\showotfcomposition{KacstOne.ttf*yeskern}{-1}{صرصرصرصر}
%\showotfcomposition{KacstOne.ttf*yeskern}{1}{صرصرصرصر}
\stoptext