Container issue for Policy-based authorization

20K PV
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This is the Container issue for this doc. Reopen each issue you are working on so the owner is notified.

- [ ] Readability #17519
- [ ] Discuss DefaultPolicy et al in "Policy-based authorization" #16147
- [ ] OAuth/OpenID Connect Scopes in the Authorization? #12999
- [ ] How is a handler that handles multiple requirements invoked? #16333
- [ ] Nice guide, but i suggest to change the order of Sections #16475
- [ ] Indicate how to specify error messaging to the user based on authorization failure #16751

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#### Document Details

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* ID: b1141d92-adeb-6841-229f-914c808f9a2b
* Version Independent ID: 7089b376-949f-c1c6-c37a-5d8cc7ecf104
* Content: [Policy-based authorization in ASP.NET Core](https://docs.microsoft.com/en-us/aspnet/core/security/authorization/policies?view=aspnetcore-3.1#accessing-mvc-request-context-in-handlers)
* Content Source: [aspnetcore/security/authorization/policies.md](https://github.com/aspnet/AspNetCore.Docs/blob/master/aspnetcore/security/authorization/policies.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-security**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**