As this is a difficult addon to introduce - there's so much in it - here is a scenario in which FCOIS works wonders. Please feel free to add your own experiences with FCOIS and suggestions to other players how to use it in the thread below.

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Addon-author is @Baertram on PC/EU
Source: https://www.esoui.com/downloads/info630-FCOItemSaver.html

Here is how FCOIS helps with Daily writs: 'run as many daily writs on several chars in the shortest amount of time incl. inventory management'

- FCOIS ' custom Lock (red) is highly useful in marking potions, food, mats. etc that are required for daily writs and that are kept pre-crafted either in the bank or crafting bag (for auto-withdrawal via Dolgubon's LWC).
The red lock prevents certain actions (configurable) such as accidentally selling or sending these items, and overall serves as good marker to check up on these items in case of having to restock them.

- 'Mark with selling' (inventory option/also as keybind) and 'Move all selling-items to Junk' (same/same): To keep a daily writ run under 2 minutes (for 7 daily writs including inventory management) these two functions of FCOIS are paramount. Items (Example: a specific green recipe) can be marked with the selling icon. Thereby any other instance of this item is automatically marked as 'selling' when it enters your inventory. With a single button press you can then move all 'selling' items to Junk. Daedra Husks, etc.,you no longer sell or move to junk individually but: open inventory, press 'move to junk' - sell at merchant, done.

- Cannot stress enough how much the latter eases your 'inventory game'. For this function alone FCOIS is fully worth the install. And this is jus the tip of the iceberg in terms of what this addon can do. (Another helpful addon for inventory, banking, junk management is Personal Assistant Banking, etc.)

- another feature used somewhat unconventionally on my part are the 'dynamic icons' (there are ~3 by default). I attached them to the three junk potions there are (essence of stamina, health, magicka) so as to distinguish them easily in the inventory from other potions that are 'selling' or crafted.

- another highlight are the 'set-markers': you can mark different gear sets with differently colored gear icons. Thus you you look at your inventory (or bank) and can quickly note: this item is for usage. But more importantly on the Equipment screen! You only need to take 1 look to check up on whether the char is wearing the gear you want it to!


So much for a very unappreciative take on this addon, because it is literally fraught with functionality (most of which I do not even know about.)
But beware: This addon will not remedy you of any obsessive inventory management urges you've picked up along the way. But it greatly reduces the opportunity for that