For l/c measurement, what frequency do this use? 100hz? 1khz? 10khz? or?

Yes, the question of frequency is an excellent one. Both capacitors and inductors have values that vary with frequency. Most LCR meters will display the resonant frequency of the component along with either 1 or 2 values. The value at it's resonant frequency and it's high and low range values over a given frequency sweep. Try to buy a torroid inductor here on Amazon. Every one has a frequency. Why?Capacitors, there are so many different kinds because they are made to be effective in certain frequency bands. If you measure a tantalum capacitor in audio range like this, then you're wasting time. They are meant for very high frequency. Look at your computer motherboard. All those yellow caps are tantalum because ceramic ones won't work in Ghz range. There is a capacitor for every frequency range. It matters big time. The right way to display capacitance and inductance is a measurement over a frequency sweep. It's like the frequency response graph of your speakers. Then, you can see if you've got the right kind and if you are building a resonant circuit then it's essential. If you pick the wrong type of capacitor for the frequency range of the piece you're working on, you'll get some strange and unstable behaviors, not what you want. So yes the question is very good. Without a spectral graph or at least its resonant frequency, the reading is absolutely meaningless, unless you are working in the exact same range as...1 Khz? I see someone measured it. I don't know personally. So that's probably good from 100 to 10K Hz. Audio stuff. It's got to specify it's resonant frequency though so you know it's behavior. Inductors come in many shapes and materials also and for the same reasons. You got to know it's internal resonant frequency so you can get the results you need. For the price here, they made the assumption you're dealing with audio frequency range. That's my guess. It's important. Very much.

What is the "dh" button on the lcr meter for?

The DH button is "Data Hold". It holds the reading on the display at the time the button is pressed for easy reading until it is pressed again. This meter works very well, with excellent accuracy. It is an excellent bargain at the price, although it does not measure inductor "Q" or capacitor dissipation.

Can you test capacitor leakage with this?

This meter is not designed to test leakage. You can infer leakage though by the fact that the meter measures capacitance by measuring the time it takes to charge the capacitor. If the capacitor is leaky it will take longer and therefor it will display a larger capacitor value that it actually is. Capacitors do not grow in capacity as they age so a reading of something larger than the value on the cap tells you it is leaky. So if you have a 50 year old wax coated paper cap you can put money on it being leaky. Measure it to see how far off the value is then throw it away and replace it with a modern part like a Mylar film cap. There are a bunch of youtube videos on measuring and replacing old capacitors.

I need to make a coil for an old Heath transceiver kit, and to trim it to get the desired impedance; can this device measure / check that impedance?

I've used this LCR meter to trim to the correct inductance in winding coils. It worked great for the lower RF frequencies I was interested in, but with higher frequencies and corresponding lower inductances, I can't verify accuracy. A once expensive lab type instrument is now easily affordable for the hobbyist.