1. Determine the Purpose of the Project: The first step in choosing the right type of square steel pipe for your project is to determine the purpose of the project. For example, if you are building a fence, you may need a different type of square steel pipe than if you are building a support structure for a building.

2. Consider the Size and Shape: The size and shape of the square steel pipe are also important factors to consider. For example, if you need a pipe with a larger diameter, you may need to choose a thicker gauge steel to ensure it can support the weight of your project.

3. Think About the Material: The material of the square steel pipe is also important to consider. Some common materials include carbon steel, stainless steel, and aluminum. Each material has its own unique properties and benefits, so it's important to choose the right one for your project.

4. Look at the Coating: The coating on the square steel pipe is also important to consider. Some coatings, such as galvanized coatings, can help protect the pipe from rust and corrosion, while others may be more decorative in nature.

5. Consider the Cost: Finally, it's important to consider the cost of the square steel pipe. While you don't want to sacrifice quality for cost, you also don't want to overspend on a pipe that is more than you need for your project.

Example: If you are building a fence, you may want to choose a square steel pipe with a galvanized coating to protect it from rust and corrosion. You may also want to choose a thicker gauge steel to ensure it can support the weight of the fence. On the other hand, if you are building a support structure for a building, you may want to choose a square steel pipe made from stainless steel for its durability and strength. 

Square and rectangle tubes can come in a variety of dimensions, ranging from small sizes used in furniture and decorative applications to larger sizes used in construction and industrial settings. The thickness of the tube can also vary depending on the intended use, with thicker walls providing greater strength and durability.

Another factor to consider when choosing a square or rectangle tube is the type of steel used. Common types of steel used for these tubes include carbon steel, stainless steel, and aluminum. Each type of steel has its own unique properties and advantages, such as corrosion resistance or high strength.

Overall, the choice of square or rectangle tube will depend on the specific application and requirements. Welded tube may be suitable for less demanding applications where cost is a factor, while seamless or extruded tube may be necessary for applications requiring greater strength and durability. The dimensions and thickness of the tube, as well as the type of steel used, should also be considered when selecting the appropriate tube for a given application.