Stereo microscopes employ several kinds of magnification systems, each with its advantages and disadvantages.
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A fixed magnification system has one pair of objective lenses and provides only one magnification level. To change magnifications you can install new objective lenses or switch to a different set of eyepieces. Fixed stereo microscopes are suitable if you only need to work with one magnification or rarely need to change magnifications.
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A turret magnification system typically has two pairs of objective lenses, allowing you to switch between two levels of magnification by turning an objective changer, which is usually located under the microscope body.

So if you had a turret changer with 2x and 4x objectives, and were using 10x eyepieces, then you'd be able to switch between 20x and 40x magnifications but nothing in between.

These are popular in many applications, including gemology, hair transplantation, and dentistry.
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A zoom magnification system provides a full range of magnification levels. It usually has zoom knobs on either side of the microscope body indicating a maximum and minimum magnification factor.* Rotating these would allow you to visit any of the intermediate magnification levels.

So if you had a minimum magnification factor of 1x and a maximum of 7x and were using 10x eyepieces, then you'd be able to view the object at any of the magnifications from 10x up to 70x, including 11x, 12x, etc. Or even 11.5x, 11.6x, etc.

Which one to get?

Fixed and turret systems lack the flexibility of zoom models, but are usually more economical and promote repeatability. This is useful if you need to view multiple samples under one set magnification level and makes it easier to determine that magnification without having to read an analog scale.

Some zoom systems address repeatability by providing "click stops" or "detent" knobs which click into place at specific magnification intervals. This makes it easy to return to a previous magnification or go back and forth between exact magnifications if your application calls for it.

* Some older models have the zoom knob under the body, like the Meiji Techno EMZ-2 shown below:

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