Regarding the classification of whether a track must be marked as "Explicit" or not, there are clear guidelines of the stores. Basically, the rule applies here that a release must be marked accordingly for words such as "fuck" or similar. This applies to both the lyrics and the song titles. Another rule of thumb here is also "What would you play to your 5-year-old child?".

To mark a track accordingly, simply select "Yes" under "Tracklist" > "Edit" > "Explicit" > "Yes" in your release.
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However, for tracks that contain "Explicit Content", some stores will restrict availability.
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Basically, most stores like Spotify, Apple Music, Amazon Music or Deezer have the option to hide explicit content in the apps. Users who have made this setting will no longer see these tracks.
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With Spotify, this additionally affects users who use the Spotify Kids tariff. However, the chances of being included in a curated Spotify playlist or appearing in the release radar of its fans are not affected by this.
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On iTunes/Apple Music, explicit content will generally not be available in the following countries: Belarus, Burkina Faso, India, Nepal and Uzbekistan.
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​Note: If the cover of your release includes the "Explicit Content" logo, at least one of the tracks included on the release must also be marked accordingly.