Customer segments should describe customers. An order line's Sales value can't tell you whether its customer is a frequent buyer, and several lines in one order don't count as several visits. Build customer-level totals with FIXED and count distinct Order ID for purchase frequency.
A parameter lets a stakeholder adjust the spend threshold without editing the calculation. State what each boundary means. A customer exactly at the threshold belongs in a segment defined as at least that amount. The order of an IF statement matters when its conditions overlap.
Sets provide a second lens. You can keep a manually reviewed audience in a set, combine it with a profitable-customer set, or use In and Out on Color without deleting everyone outside the audience. When you summarize any of these segments, count distinct customers. Summing a repeated customer total at the order-line grain counts the same person many times.