What is the unit of elevation in glo_30 Digital elevation map data

Question

I'm trying to learn how to create elevation contour lines on a map in python following some of the ideas in https://medium.com/analytics-vidhya/creating-contour-lines-on-folium-map-with-python-b7994e67924b .

Using https://pypi.org/project/dem-stitcher/ I have the following code to download DEM data within a bounding box:

bounds = bounding_box_naive(coordinates)
print(bounds)
flattened_bounds = [coord for point in bounds for coord in point]
print(flattened_bounds)


X, p = stitch_dem(flattened_bounds,
                  dem_name=dem_name,
                  dst_ellipsoidal_height=dst_ellipsoidal_height,
                  dst_area_or_point=dst_area_or_point)

I then wrote it to a file with:

fig, ax = plt.subplots(figsize=(8, 8))
ax = plot.show(X, transform=p['transform'], ax=ax)
ax.set_xlabel('Longitude', size=15)
ax.set_ylabel('Latitude', size=15)

followed by:

height_type = 'ellipsoidal' if dst_ellipsoidal_height else 'geoid'

with rasterio.open(out_directory / f'{dem_name}_{height_type}_{dst_area_or_point}.tif', 'w', **p) as ds:
    ds.write(X, 1)
    ds.update_tags(AREA_OR_POINT=dst_area_or_point) 

I eventually end up with data in a dataframe that looks like:

    longitude   latitude  elevation
1   -88.432778  31.661389  72.399994
2   -88.432500  31.661389  74.471750
3   -88.432222  31.661389  76.551100
4   -88.431944  31.661389  76.705060
5   -88.431667  31.661389  77.992940
..         ...        ...        ...
212 -88.431944  31.654167  79.082660
213 -88.431667  31.654167  79.882225
214 -88.431389  31.654167  78.264725
215 -88.431111  31.654167  79.834656
216 -88.430833  31.654167  79.141390

[216 rows x 3 columns]

I can understand the latitude and longitude, but what unit is the elevation in?