In h5py, I can use the following code to open a file

```
f = h5py.File('myfile.hdf5','w')

```

According to the doc,

```
w   Create file, truncate if exists
w- or x Create file, fail if exists

```

What does "truncate" mean when using "w"? Generally speaking, what are the different scenarios of "using w" vs "using w-"?