Because Austen very clearly presents Elinor as the respectable
heroine of the novel, she also becomes the model for proper
behavior in this time period. Marianne is the opposite, and
often does/says things that would be considered improper. If
you note Elinor's behavior in social settings, it will help you to
discover the specifics of what counts as proper behavior.
Some examples are she rarely turns down invitations unless a very
good reason exists, she makes it a point to continue casual
conversation with visitors even when it is painful or awkward, she
refrains at times from sharing her opinions if she knows they will
not be well received, and she avoids being alone one on one with a
man if possible.
Again, using Elinor as an examples, Austen does present the idea
that there are moral constraints on what should be said, asked, or
known. Throughout the novel, Elinor is curious about Edward
and his family interactions, but in many situations she refrains
from asking questions because she feels it would not be
right. She keeps Lucy's secret about the engagement even
when it is painful because she has given her word. Mentally
and verbally, Elinor scolds Marianne and Mrs. Jennings for being
too free with their opinions and questions. When Willoughby
comes to confess towards the end of the novel, Elinor is very
uncomfortable and attempts to leave several times, feeling that she
should not be hearing such an open confession.