The honest version of "does it integrate with my software." These are the systems this industry actually runs on, and what each one means for a build.
Epicor Prophet 21Distribution ERP
A dominant system of record in mid-market distribution — orders, inventory, pricing, purchasing and customer history. Integration is workable and well-trodden, which makes it a comfortable place for an agent to both read and write.
NetSuiteERP
Common where the distributor grew up on cloud systems or has multiple entities. A strong, well-documented interface, and the easiest of this group to build against.
Acumatica / SAP Business OneERP
The other two you meet regularly. Both integrate, with different amounts of ceremony; the deployment matters more than the product name.
EDITrading partner integration
SPS Commerce, TrueCommerce and the rest. Handles the standard transactions well and fails in a specific way that costs real money — an order rejects for a mapping reason, sits in an error queue, and nobody is watching the queue.
Customer portalsOrder intake
Larger customers require you to log into their system rather than sending an order. Manual, daily, and rarely integrated with anything.
Warehouse managementWMS
Either an ERP module or a separate system. Where picking, receiving and cycle counts live, and where the discrepancy between what the system thinks you have and what is on the shelf becomes visible.
Supplier price filesPurchasing
Price updates arriving as spreadsheets, PDFs and portal downloads on each supplier's own schedule. Getting them into your pricing before they reach your margin is a recurring, deadline-driven manual job.
Freight and carriersLogistics
Rating, tracking and freight bills. Freight billed versus freight quoted is a persistent quiet leak in most distributors.
Sales CRMCustomer management
Where outside sales activity lives, when it lives anywhere. Frequently disconnected from the order history that would make it useful.