It is an industry based on groundbreaking new technologies, where in a handful of well-established corporations sell highly specialized devices for business use and a fast-growing number of start-up companies produce innovative toys, gadgets for hobbyists and other interesting niche products. 

请问where引导的定语从句中,in是什么词性？是什么用法？

我觉得没有in也很通顺啊，主语a handful of well-estabilished corporations,谓语sell,宾语specialized devices for business use.

反倒加一个in我就不知道该怎么理解了。所以请教一下各位老师。