There is no clear definition of a boundary between space and not-space. Some like to use the definition of "space" as "no-meaningful atmosphere". Note that the atmosphere does extended very far indeed into what we typically deem "space". Typically we model gases (like the atmosphere) as discrete packets of continuous properties. If density is below a very low threshold, we can't do that, and have to model gases using rarified models (where gas behaves more like individual particles). If we use the 1976 Standard Atmosphere model this happens at above 86km. Therefore to atmospheric scientists, space is 86km.
Now let's say you're building an orbital space plane. To stay aloft, your gravity opposing forces come from two sources: aerodynamic lift and centrifugal force. When you go faster, both forces increase, so for every altitude, there's a speed which keeps you aloft. However lift force is a function of density, and density drops with altitude. Therefore there is a point where aerodynamic lift is essentially meaningless compared with the centrifugal. At this point, you are more of a orbital vehicle than an aerodynamic one, so we can say that you are "in orbit". This altitude is The Kalman line. The important thing to note is that the Kalman Line is a definition prescribed to a specific vehicle (the Bell X-2, although it is rarely discussed in that context). Changing the weight and lift properties change it. Hence the boundary is very fuzzy and the exact source of definition changes depending on how it's calculated. It's also been calculated based on aerothermal heating considerations, and based on the operational limits of air-breathing engines. Therefore, it's been cited as 83km, 86km, 90km and 100km depending on the specific source. A key takeaway here is that The Kalman Line is a speed as well as an altitude, so it doesn't even make sense to use this as a boundary in a suborbital context. But for orbital scientists/engineers, you safely say that's space is not below 80km, and certainly above 100km.
Now imagine you are the US Air force and some of your pilots flew an experimental aircraft (the X-15) 90km altitude, and you want to put astronaut wings on them to gain some inter-service rivalry clout. Well for you, the definition of space is then 90km.
Now let's say you are the FAA and want to quantify where your jurisdiction ends. Well, pick an arbitrary definition based on Kalman's notes. 100km should be good enough for your needs.
Now let's say you are a satellite engineer and want a definition where your satellites will actually stay in orbit for a few trips around the earth without thrust corrections. Pretty hard to do at 80km, there's too much air. As it turns out the lowest sustained circular orbits occur at around 125 km. So 125 km is a good definition of space for you.
There's some much needed historical context. Now let the useless arguments surrounding the pedantry of fuzzy definitions continue.