Max login time sets a timer to automatically disconnect anyone who has been logged in regardless of what they are doing after so many minutes.
Max idle time starts a timer to disconnect someone anytime they aren't transferring a file, or getting a directory listing.
Max upload and download speed will limit a user's bandwidth to the specified amount.
Download to upload ratio allows you to force people to upload files to you before they can download from you. For example, every 10 Mb they upload to you, they can download 1Mb.
Max upload individual size controls the max size of a file that a user can upload. This may be useful if you don't want users storing large files on your server.
Max upload/download amount per session allows you to control how much a user can transfer before they are blocked, and they must disconnect and re-connect.
Max upload/download amount per day, or month lets you control the total amount users can transfer into your server for that time period.
Disable partial downloads will prevent a user from downloading part of a file if Crush calculates they won't be allowed to finish the download because of one of these restrictions.
Dir to use for quota lets you configure a parent dir for quota calculations. The purpose of this is to have users who are all member of a group, and they all share the parent users quota.