10GBASE-SR is a port type for MMF (multimode fiber) and uses 850 nm lasers.
For longer reach, another standard defined in IEEE 802.
3ae is 10GBASE-LR.
10GBASE-LR is a port type for SMF (single mode fiber) and uses 1310 nm lasers.
Traditionally SMF 10GBASE-LR SFP+ modules are way more expensive than MMF 10GBASE-LR SFP+ modules.
MMF and SMF historical short range / long range paradigm may change in short term: since 2014, for new installations, especially indoor, SMF wiring is about half of MMF wiring.
As a consequence, lots of new buildings and datacenters now standardize on SMF.
But 10GBASE-LR SFP+ transceivers are too expensive for short-range connections.
A new market for short distances over SMF has been created: the industry is standardizing on a new class of SMF 10G transmitters with typical 10GBASE-LR LITE SFP+ transceivers.
While not a standard yet, 10GBASE-LR LITE has a specified reach of 2 km over standard 9µm single mode fiber.
10GBASE-LR LITE isn’t a replacement for 10GBASE-LR optics: it is just relevant alternative for “shorter” reach SMF applications.