What is the meaning of "a5ultexx" in terms of phone models? I have an sm-a500fu 2015 and I've been tinkering with it for the last few months, custom recovery and root. 

I've found a ROM port for it which I want to install and the latest version is for the ROM is for a model a5ultexx. The previous versions of the ROM explicitly say a500fu but the latest says a5ultexx. Does this mean my a500fu is compatible, or that all a5 models are compatible?
a500fu or a5ultexx are codenames, but not necessarily ones given by the manufacturer. ROM maintainers will determine a suitable codename representative of the device in the bringup process, and that codename can change during the device's life cycle (in another answer of mine, I described a similar change to another device).
For recent Samsung devices there's an undocumented (AFAIK) pattern in the codenames. Take your device for example:

a5ultexx = SM-A500FU

a5 is obviously Galaxy A5
u for sub-variant, the &quot;U&quot; in &quot;A500FU&quot; - this part is rarely seen
lte for LTE variants, versus wifi for WiFi-only tablets, or 3g for 3G variants.
xx for general region/carrier, in this case, for international devices, the &quot;F&quot; in &quot;A500FU&quot;.


As you can see the 2 expressions point to the same device, so yours is compatible.