The linked document kinda sorta explains it, by saying that Span is a by-ref type. Let me explain what this actually means.

In .NET (below C# level, in the VM itself), there are two fundamental types of pointers - managed (e.g. int&), and unmanaged (e.g. int* ).

An unmanaged pointer is basically the same as a C pointer. It points to whatever you tell it to point, and it's your responsibility to ensure that it's still valid when you dereference it. So you can get dangling pointers to locals that went out of scope, for example. If you point it at some memory that is managed by GC, you also need to make sure that GC doesn't automatically move that memory, because the pointer will not be updated - the VM provides an opcode for "pinning" managed objects so that they don't move to facilitate this. Also, because these pointers are "dumb", you can do pointer arithmetic on them, cast them to/from integer types, etc. And they can be used in any position a valid type can.

A managed pointer, in contrast, is a pointer that is guaranteed to be memory-safe. For managed pointers that reference managed objects and their fields, this means that GC is aware of those pointers, and adjusts them as it moves the objects around in memory, just like it adjusts regular object references (so you don't need to pin anything; things "just work"). When you have a managed pointer to stack-allocated data, the VM basically makes it illegal to return such a pointer, or stash it away into a variable that can outlive the scope - this is enforced by the bytecode verifier, simply by prohibiting fields of managed pointer types in heap-allocated objects (including, recursively, in any structs). So the only legal operation that you can do with a managed-pointer-to-local is to pass it into a function call - since the stack frame of the calling function is guaranteed to be there for the duration of the call, that is memory-safe.

On C# level, unmanaged pointers are just pointers (int* ), and managed pointers are used to implement ref types, as in "void Foo(ref int x)". Until recently, function arguments were really the only place they were permitted, so they were only used to pass arguments by reference. Recently, they've also added the ability to declare ref locals and return by reference, subject to all the verification rules - e.g. if you return a ref, you cannot return a ref to a local, it must be a ref to a field, or a ref argument that you got from the caller.

The runtime additionally has some types, that effectively wrap a managed pointer and add some functionality to it. One existing example is TypedReference (https://docs.microsoft.com/en-us/dotnet/api/system.typedrefe...) - this is basically a type-erased managed pointer, plus runtime type of whatever it points to. You can then "downcast" it to a proper typed managed pointer, and because it knows the target type, it can verify that the cast is valid. There's also a type called ArgIterator, which is basically the .NET type-safe equivalent of C's va_list. These are used for various low-level stuff like C++/CLI vararg functions.

Now, these wrapper types, because they encapsulate a managed pointer, have all the same verification restrictions that managed pointers themselves have. And all these types, including managed pointers themselves, are collectively called by-ref types.

Now, Span<T> is basically just a new by-ref type, that combines a managed pointer with span length under the hood. As such, it's subject to all the same restrictions, which together are sufficient to make sure that it's not possible to use it in such a way that it points to invalid memory.