Question about California LLC
Practically every state requires an LLC to maintain a Registered Agent on file. Registered Agent is an organization or an individual that resides in the state and is available during business hours at that address.
Kit & Seal are more like optional items nowadays. Most of the time you won't need your seal, although in some cases you do need it. Examples include buying a surety bond, which is required for certain types of licenses, or if you do business internationally many countries (like Russia) require you to seal the contracts with a company seal. In general, in the US sealing internal company documents is no longer required as long as they are signed, but I would say having it also sealed makes it look really good.
Kit is a professional company folder where you can keep all your company documents, and it usually comes with stock certificates and a compliance CD which is pretty handy to make sure all your internal documents are in place. For the price you pay for those company kits come very handy if you have several companies, so that you can store all the corporate paperwork in each Kit and have easy access to all your documents.