Can I eat it only in Hat Yai?
No. Hat Yai-style fried chicken is sold in other areas too. If you have come to Hat Yai, you could try the dish known by the name of the city with sticky rice.
Hat Yai Fried Chicken · ไก่ทอดหาดใหญ่
In Hat Yai, try crunchy fried chicken with fried shallots on the side. It is Hat Yai fried chicken, known by the name of the city. The richness and aroma of the shallots are added to the crunchy chicken, and it goes well with sticky rice. Put a little of the shallots served with it on top of the chicken and take a bite.

It is a dish in which you enjoy the crunch of the fried chicken and the rich taste of shallots together. It is also characterised by seasoning with aromatics such as garlic, coriander root and white pepper. You can also try the chicken and shallots together before adding any sauce.
Eat the fried shallots together with the chicken to enjoy the richness more.
Hat Yai is a city in the Songkhla area, where the food culture of Chinese-descent and Muslim communities sits together. Hat Yai fried chicken is known as a signature dish of this area. It is sold in other cities too, but if you have come to Hat Yai, have it as a meal with sticky rice.
No. Hat Yai-style fried chicken is sold in other areas too. If you have come to Hat Yai, you could try the dish known by the name of the city with sticky rice.
No. The kind of meat alone cannot decide whether a dish is halal. If you need a halal meal, look at the restaurant's certification together with the actual ingredients and cooking conditions.
If you want to read more about these dishes and regions, these sources are a good place to start. Ingredients and cooking methods can differ from one eatery to another.