The idea in one line. An assortment is not a list of products you like. It is a system of roles, widths and depths that states, in stock, who your customer is and how she shops.
Why it matters. Every merchandising failure you will meet — dead stock, empty shelves, markdowns eating the margin — began as an assortment decision that nobody wrote down. If you cannot say what job each product does in the range, you cannot say why it is there, and the market will decide for you at 50% off.
Three dimensions. WIDTH: how many distinct options (styles, colourways) you offer. DEPTH: how many units of each. ROLE: what each option is for. Width creates choice and risk; depth creates availability and cash exposure; role decides which of the two an option deserves.
Product roles. CORE — never out of stock: the items that pay the rent, proven sellers, bought deep and replenished, low fashion risk; the customer expects to find them. FASHION — the season's argument: what makes the range current, bought shallower and timed to the season; this is where your trend work lands. IMAGE — the hero: pieces bought to be seen more than sold — the window, the press, the reason to walk in; bought thin on purpose. The ratio between the three is a strategic choice, not an accident: roughly 70/25/5 (core/fashion/image) is a stable shop; 30/60/10 is a fast-fashion bet.
The bridge to trend work. A signal you classified as CLASSIC belongs in core. A TREND with independent carriers belongs in fashion with real depth. A FAD gets image treatment — thin, fast, with an exit date. Merchandising is where the carrier test turns into units.
The trap. Buying by taste inflates width: every option seems worth having, so none gets the depth to sell through. The customer reads a wide, shallow range as "nothing in my size". Width is the most expensive luxury in retail.
Practice. Take any brand's current range, or your own project. Sort every option into core / fashion / image and estimate width and depth per role. Then answer in one sentence: what does this system say about who the customer is? If the answer is "everyone", the range has no system.