It can be a topic that you are passionate about and know enough to be considered well versed in your field or it can simply be a commercial decision based on how you would like to make money.
Maaari itong maging isang paksa na ikaw ay madamdamin tungkol sa at sapat na itinuturing na mahusay na dalubhasa sa iyong larangan alam o maaari itong simpleng magiging isang komersyal na desisyon batay sa kung paano mo nais na gumawa ng pera.