When reading about standing waves it is always said that only certain wavelengths are "allowed". I understand that these wavelengths are a requirement for there to be a standing wave due to the boundary conditions, but what does "allowed" mean in this context?

When creating a wave on a fixed string, does this mean that the wavelength will always be an appropriate fraction of the length of the string such that a standing wave exists - i.e it is not possible to create waves with a wavelength that would not create a standing wave on a fixed string?

Or are the wavelengths completely dependent on the source that created the wave, and standing waves are simply the special case/coincidence when the wavelength is appropriate?