Start with how you actually price
B2B projects tend to begin with a brief about the storefront and discover, several weeks in, that nobody has written down how pricing actually works.
Not the official version — the real one. Which customers are on which contract, what happens when a contract lapses, who can authorise an exception, and whether the number in the ERP is the number that gets invoiced. Until that is explicit, no storefront can display a price with confidence.
That mapping is the first phase for a reason. It is also the part that tends to be genuinely useful to the business regardless of what gets built afterwards.
The portal is the product
For repeat B2B customers the storefront is not a shop window. It is an operational tool they use weekly, and they judge it the way they judge any tool: can I find the thing I bought last time, is the price right, when will it arrive, and where is my invoice.
Merchandising matters far less than it does in retail. Reordering, order history, saved lists and self-serve invoices matter far more. Designing a B2B site like a D2C site is the most common and most expensive mistake in this category.
Where this sits
This is a specialisation of eCommerce development rather than a separate discipline, and it depends heavily on integration work because the pricing authority is nearly always the ERP. If you are moving from a legacy B2B platform, the sequence is covered under platform migration.