[wolrah]

Depends on the model, or actually technically it depends on the bootcode but which version is preloaded depends on the model.

The brand new Pi 3B+ ships with the latest bootcode and will boot from USB mass storage or PXE by default. Their PXE is a bit nonstandard but it's close enough that anyone who's PXE booted a PC will understand it.

The older Pi 3 supports USB and PXE booting, but has those modes disabled by default. There's a bit you can set in one-time programmable memory to enable it permanently. PXE booting has some quirks in this mode and doesn't get along with switches that take more than a second or so to activate ports.

Earlier models do not support anything but SD on their stock bootcode.

In all cases, a SD card containing nothing but the latest bootcode can be inserted which brings the new features to the older models, allowing them to boot the rest of the OS from whatever.