Employees always (in my experience) are explicitly restricted from trading company stock based on non-public info. There are also [usually] specific windows in which employees cannot trade in the company's stock.
Additionally, I have worked for companies which prohibited transactions in any derivatives or options and barred shorting the stock.
Certainly, I too was restricted at my company: no shorting (of any stocks!) and no buy/selling of stocks x-days before earnings announcements. A part from that, you could trade without any other contractual restrictions. The determination of material information wasn't not decided by the company... and I guess it's something that is is only brought up in case of an investigation.
Additionally, I have worked for companies which prohibited transactions in any derivatives or options and barred shorting the stock.