TSLICE sets the size of the thread listing "slice" (ie. a portion of the complete thread listing) when the $TSLICE$ resource variable is used. The value of the TSLICE resource is two numbers separated by a colon. The first number is the number of messages listed before the current message in the thread slice. The second number is the number of messages listed after the current message.
The default setting causes a thread slice to start with the current message. However, you may want to have the current message in the middle so the reader can see message that come before along with after the current message. The following causes the previous two and next two messages to be listed, with the current in the middle:
<TSlice> 2:2 </TSlice>