showing the usage of - before a property rule

test.py

from kivy.app import App
from kivy.lang import Builder
from kivy.factory import Factory
from kivy import properties as P


KV = '''
<CustomLabel@Label>:
    font_size: app.font_size1

<CustomLabel2@CustomLabel>:
    font_size: app.font_size2

<CustomLabel3@CustomLabel>:
    -font_size: app.font_size2

BoxLayout:
    orientation: 'vertical'

    CustomLabel:
        text: 'CustomLabel'

    CustomLabel2:
        text: 'CustomLabel2'

    CustomLabel3:
        text: 'CustomLabel3'

    Slider:
        value: app.font_size1
        min: 0
        max: 100
        step: 1
        on_value: app.font_size1 = self.value

    Slider:
        value: app.font_size2
        min: 0
        max: 100
        step: 1
        on_value: app.font_size2 = self.value
'''

class Application(App):
    font_size1 = P.NumericProperty(10)
    font_size2 = P.NumericProperty(10)

    def build(self):
        return Builder.load_string(KV)


if __name__ == "__main__":
    Application().run()