What is lead-in termination?
For Un-buffered DIMMs (UDIMM) the CLK, CTRL and ADD/CMD topologies are 
divided into two segments  The segments between the 
connector and the first DRAM are termed as the lead-in section while the segments 
between the first DRAM and the last DRAM to the termination are termed as the 
loaded section.  In order to reduce the impedance mismatch seen at the first DRAM, 
the lead-in section is a lower impedance trace (typically 40 ohms) while the loaded 
section is 60 ohms.  For Registered DIMMs (RDIMM) the post register nets are typically routed with same trace impedance.

From Micron material