When a clothing designer plans to start a business in apparel and clothing, she first comes up with a clothing line that she will cover for the design development. In the same way, when a business owner starts a fashion brand (in apparel industry), they also come up with a specific clothing line.
So you see, the term "clothing line" is used to refer to a specific product group or a product range developed for a specific season and for the specific market segment. The clothing line and product line are used interchangeably.
A source defined the term ‘Clothing Line’ as below:
A clothing line is a collection of clothing items developed by a clothing brand for a specific target market. A clothing line is a way for a brand to showcase its style and creativity within a specific product range.
A clothing line is the range of apparel items offered by the clothing company. The lines are usually referred to by season. There is a fall line, a holiday line, a spring line, and a summer line. The ‘lines’ can be for product groups like tops, bottoms, sweaters, or jackets. It just depends on what the company offers. The same clothing brand can have multiple clothing lines at the same time, targeting to multiple market segments.