From Toradex tutorial:
Typically, a device tree is defined at multiple levels and composed of multiple device tree files. Device tree files (dts and dtsi) may include other device tree files known as includable device tree files (dtsi). In this manner, a board level device tree file (dts) generally includes a SoC level device tree file (dtsi). To support the modular approach of Toradex products, our device tree files usually have three levels of inclusion: carrier board, module and SoC level. This is also reflected in the device tree file names, which are composed by the three levels: ${soc}-${module}-${board}.dtb
Examples:
imx28-eukrea-mbmx283lc.dts
imx28-eukrea-mbmx287lc.dts
imx6dl-wandboard.dts
imx6dl-wandboard-revb1.dts
imx6q-nitrogen6_max.dts
imx6q-nitrogen6_som2.dts
imx6q-nitrogen6x.dts