Mini-STX
Mini-STX (aka Mini Socket Technology EXtended, Intel 5x5) is a computer motherboard form factor that supports upgradable processors. The motherboard size is 147×140 mm, which is between Nano-ITX motherboards and Mini-ITX boards that are popular in small form-factor builds. 

ASRock extended it by two inches to add an MXM slot creating Micro-STX.

Comparisons with NUC 
When compared with NUC, the Mini-STX form factor:
 Has 3/2 the motherboard area of a NUC.
 Has processor sockets, allowing for users to upgrade or replace processors.
 Offers the possibility of dedicated GPUs via MXM (on some models produced by ASRock).
 May have more I/O options due to the larger space available on the back of the board.

Comparisons with Mini-ITX 
When compared with Mini-ITX, the Mini-STX form factor:
 Has a motherboard area 3/4 the size of Mini-ITX.
 Is usually limited to SO-DIMMs, rather than full-size DIMMs.
 Lacks a full-size PCIe expansion slot, limiting users to MXM GPUs or less powerful, integrated GPUs and APUs.
 May have less I/O options due to the reduced space available on the back of the board.
 May use a 2-in-1 SATA+power cable instead of separate SATA and power cables.
 Uses a DC power connection instead of a 24 pin power cable.

References 
 http://www.pcworld.com/article/2975764/components/intel-unveils-ultra-tiny-fully-upgradable-5x5-mini-pc-motherboards.html
 http://www.intel.com/content/www/us/en/technology-provider/products-and-solutions/desktop/mini-stx-for-mini-pcs.html
 http://www.asrock.com/mb/Intel/H110M-STX/
 https://www.asrock.com/nettop/AMD/DeskMini%20A300%20Series/ (AMD AM4)
 https://www.asus.com/Motherboards/H110S1/
 https://www.asus.com/Motherboards/H110S2/
 https://www.asus.com/Motherboards/Q170S1/
 https://www.gigabyte.com/Motherboard/GA-H110MSTX-HD3-rev-10#ov

Computer hardware
Motherboard form factors