So, not exactly the same meaning as 'packed lunch' which might be in a bag or a box or other container.
Kind of. There's some overlap depending on the context. If I say, "I brought a sack lunch," chances are good I do mean that it is in one of those ubiquitous brown paper bags. If it's in a lunchbox, I would just say, "I brought my lunch."
However, if I am attending a conference, for example, the flyer may say "Lunch will be provided," which likely means a serve-yourself low-cost catered spread, anything from salads and soups, to barbecue, to pizza. But if it says "A sack lunch will be provided," it means premade sandwiches packaged up in bundles with a cookie and a bag of chips (crisps,) or somesuch. You might get to choose between the ham sandwich bundle or the turkey sandwich bundle, but there won't be a lot of variety. Notably, in this case the sack lunch will almost certainly
not be in a brown paper bag, but rather a plastic or styrofoam container of some sort. Maybe even a white paper bag. The sandwiches were still put together by a professional company, and professional companies don't use brown paper bags, because that's what kids take to school.
Except they don't, really. Kids take fun colorful lunchboxes. By the time they are too cool for awesome lunchboxes, and might in theory want it in a nondescript paper bag instead, they are just buying their lunch from Taco Bell or whoever else has contracted to sell fast food on campus.
