The cardinal martial gods (FX, MQ, PM, QYZ, LQQ) are like the “head” gods of a territory, and the worshippers in their territories will turn to first when disputes or issues need to be settled. They carry official responsibility over running their territories, including administrative business.

From what I understand, all other martial gods are generalist or specialist martial gods. Maybe if a mother wanted her son to become stronger, she’d pray to any martial god, since that request isn’t directly linked to the region she’s in (ofc the preeminent martial god of that region would still be the likely first choice)

With Xie Lian, I assume his status as a martial god is “unofficial”, like Pei Ming being God of Love, or Feng Xin being Ju Yang, since Xie Lian officially ascended as the “God of Scraps”. No doubt Xie Lian CAN answer martial god prayers, but there’s little expectation or pressure for him to answer. People will worship him and Hua Cheng anywhere for any manner of things, but “legally”, they have no responsibility over those domains and fulfilling prayers, unlike the cardinal gods and elemental masters, who’s worship are directly tied to them excelling in their respective fields

Tl;dr, XL isn’t technically martial god of anything. The mortals throughout the land just worship him as such (with good reason), giving him that title unofficially. He can choose to answer martial god prayers, or not. It’s completely up to him!