SysML identifies two [EDIT:ADDITIONAL] usage patterns for ports, one where ports act as proxies for their owning blocks or its internal parts (proxy ports), and another where ports specify separate elements of the system (full ports).

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It is NOT true in SysML1.6+ that you "should" always use a SysML FullPort or a SysML ProxyPort instead of a "standard" Port, and it's not even always a good idea (and the spec states this very clearly in multiple places)!
Some valid FullPorts from the outside
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OMG Systems Modeling Language (SysML) 1.6
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About Object Management Group copyright in text extracts quoted from OMG specifications for educational purposes
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Port
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ProxyPort
FullPort
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SysML identifies two [EDIT:ADDITIONAL] usage patterns for ports, one where ports act as proxies for their owning blocks or its internal parts (proxy ports), and another where ports specify separate elements of the system (full ports).
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Both are ways of defining the boundary of the owning block as features available through external connectors to ports.
The [EDIT:ADDITIONAL] has been inserted by Dr Darren here, who says:
I can't think of any sentence in the SysML-1.6 specification that causes more confusion for beginners than this one, because (together with the annoying fact that MagicDraw/Cameo by default hides some features for "standard" Ports in the 'systems engineering' perspective mode) it gives the impression that - just because SysML introduces two additional and more specific port types - users somehow "should" always use one of them. Nothing could be further from the truth.
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Webel Best Practice: It is NOT true in SysML1.6+ that you "should" always use a SysML FullPort or a SysML ProxyPort instead of a "standard" Port, and it's not even always a good idea (and the spec states this very clearly in multiple places)!
If you are a SysML OMG RTF voting member please add your support for the inclusion of the one clarifying word 'additional' in the quoted sentence here:
[SYSML17-293 ] Introduction to 9.1.3 Proxy Ports and Full Ports might be interpreted as SysML only permitting Full and Proxy Ports (not standard ports)".
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Both are ways of defining the boundary of the owning block as features available through external connectors to ports.
Proxy ports define the boundary by specifying which features of the owning block or internal parts are visible through external connectors, while full ports define the boundary with their own features.
Proxy ports are always typed by interface blocks, a specialized kind of block that has no behaviors or internal parts.
Full ports cannot be behavioral in the UML sense of standing in for the owning object, because they handle features themselves, rather than exposing features of their owners, or internal parts of their owners.
Ports that are not specified as proxy or full are simply called “ports.”
In either case, users of a block are only concerned with the features of its ports, regardless of whether the features are surfaced by proxy ports, or handled by full ports directly.
Proxy and full ports support the capabilities of ports in general, but these capabilities are also available on ports that are not declared as proxy or full.
Modelers can choose between proxy or full ports at any time in the development lifecycle, or not at all, depending on their methodology.
Modelers have the option of applying stereotypes for proxy and full ports to indicate whether ports are specifying features of their owners and internal parts (proxy), or for themselves separately (full). This is a concern when defining ports, rather ...
Using existing blocks with ports only requires knowing the port types, because they define the features available for linking or communication with those ports via connectors.
The stereotypes of proxy and full ports might be elided in these cases to simplify diagrams.
Modelers can apply stereotypes for proxy and full ports at any stage of model development, or not all if the stereotype constraints are not needed.
Figure 9-7 happens to use unstereotyped ports on a general block distributed to users, and stereotyped ports on its specializations for implementation, but the modelers might have not used stereotypes at all, if they did not care whether the model met ...
Unstereotyped ports do not commit to whether they are proxy or full, and do not prevent or dictate future application of the stereotypes, except for ports that violate constraints of the stereotypes.
For example, if the port types on the general block in Figure 9-7 had behaviors defined, then the proxy specialization would be invalid. If the general ports had binding connectors to internal parts, then the full specialization would be invalid.
Unstereotyped ports have the basic functionality of stereotyped ones, including flow properties and nested ports, so they can be used as long as the modeler is not concerned with the distinction between proxy and full, and the constraints they impose.
If the general ports had both behaviors and internal binding connectors, then both specializations would be invalid.
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