You can upgrade your HP EliteBook 8460p Laptop to up to a maximum memory capacity of 16GB Memory.
DDR3L is a dual voltage capable memory DIMM, which supports operation at both 1.5V and 1.35V. DDR3L is also pin-compatible with DDR3!
[https://www.minitool.com/lib/ddr3-ram.html|DDR3] is a single voltage capable memory DIMM, which supports 1.5V operation only.
Basically you can use either.