Standard or Weighted? What are the uses for each?

Standard Tuning Forks
Standard tuning forks are great for vibrational healing such as sound toning, and for working with energies, such as opening energy blocks, clearing chakras, or cleansing auras. In the process of promoting the flow of vital life force within the human body and the human energy field, they are intended to assist and facilitate clearing, balancing, and energizing the energy body in a simple and gentle way.

Standard tuning forks also have the unique quality of being able to produce harmonic intervals (multiple pitches that occur at the same time) when struck together.

Weighted Tuning Forks
Weighted tuning forks don't produce harmonic intervals as easily, however they have a much stronger vibration, making them easier to feel, and excellent for uses such as acupuncture, sound puncture, and muscle relaxation techniques. In addition to their own sound and vibrational healing qualities, weighted tuning forks can physically transfer their frequency of vibration to the body by using forced resonance, or by directly applying the handle (also called stem) of the tuning fork on the body at point of focus, such as a meridian or chakra. This is a way of creating movement or frequency vibration in any area of the body, and as such, weighted tuning forks are known to give immediate relief from pain when used with interferential therapy techniques.

Weighted tuning forks vibrate for longer periods of time, and stay at their exact frequency longer. This makes them great for accurate or precision tuning or toning when doing sessions on yourself or others.

Guidelines
If you are working in the etheric field, or on a fragile or debilitated person, use standard forks. If you are working on a physically strong person, and they are dealing with a physical issue, use weighted forks.

Having said all this, weighted forks are possible up to about 512 Hz. As the frequency increases, the length of the fork decreases. When the tines get too short, the resonance drops to an unacceptable level, making the fork ineffective.

Conversely, the lower the frequency, the longer the fork. For that reason, frequencies below about 50 Hz are only available in the weighted style.