I had this question in my mind for years. Based on some experience, some good books (listed in good reads) and my constant interaction with some architecture geniuses at work, I now have clarity on this :-)
Generally speaking, architecture is a branch of design. So every architectural work is a kind of design, but the reverse is not true. Not all design is architecture. Architectural designs addresses a specific goal. The goal almost always is to assure that the system meet its quality attributes and behavioral requirements. The architect creates design that manifest significant decisions and constraints in the system’s architectural structure. There are also design decisions, that are not architecturally significant, i.e. does not belongs to the architecture branch of design. We can view them as some component’s internal design decisions, like- choice of algorithm, selection of data structure etc. Any design decision, which isn’t visible outside of its component boundary is a component’s internal design and is non-architectural. These are the design decisions a system architect would leave on module designer’s discretion or the implementation team. The implementation team is free to design their subsystem as long as their design don’t break the architectural constraints imposed by the system level architecture.
Architecture can be detailed or descriptive, such as defining a protocol that will be used by outside world entities to interact with the system (e.g. A hypermedia RESTful interface for a service of a large system), OR it may be some constraints defined at high level to support quality attributes and to be followed by the system’s internal implementation (e.g. The usage information of all the 6 resources should be collected every 15 seconds and stored in-memory in binary format with memory usage of no more than 16 bits per sample; The usage info buffer should be uploaded to server every 8 hours or whenever the coverage is available after 8 hours).
Architectural design deals with the quality attributes (performance, security, changeability, reliability, user experience etc.) and behavioral requirements (e.g. Usage information of devices should be upload to server with least possible network footprint) of the system/element. Anything that needs design but is not in the above two categories, IS element’s internal design AND NOT system’s architectural design. However, module designers may have to introduce structure or may have to take design decisions to fulfill the behavioral requirements of their module. In that case, those design decisions are architectural to their module, but not to entire system. Hence, design decisions to be called as architectural or not, is a function of the context.