In the manifest.py you can specify if it is backend or frontend like below:

'assets': {
    'web.assets_backend': [
        'bus/static/src/**/*',
    ],
    'web.assets_frontend': [
        'bus/static/src/js/longpolling_bus.js',
        'bus/static/src/js/crosstab_bus.js',
        'bus/static/src/js/services/bus_service.js',
    ],
    'web.qunit_suite_tests': [
        'bus/static/tests/*.js',
    ],
},