Axial resolution refers to the ability to distinguish two structures that are close to each other along the direction of the sound beam. It is determined by the spatial pulse length (SPL), which is the product of the number of cycles in a pulse and the wavelength. Shorter SPL means shorter pulse duration, which leads to better axial resolution. This is because shorter pulses allow for better separation of echoes from closely spaced structures.
Reference:
ARDMS Sonography Principles and Instrumentation guidelines
Kremkau, F. W. (2015). Diagnostic Ultrasound: Principles and Instruments.