I think it means exactly what it sounds like. My mom loved the name [name_u]Abby[/name_u] and liked the name [name_f]Abigael[/name_f]. She named me [name_f]Abigael[/name_f] to give me options for when I become an adult, but I’ve gone by [name_u]Abby[/name_u] since the moment I was born. If I felt as though [name_u]Abby[/name_u] was too cutesy or nickname-y for me, I had the option of a longer name in [name_f]Abigael[/name_f]. As it is, I don’t feel like my nickname is super cutesy and, despite the fact that a future nameplate of mine will read [name_f]Abigael[/name_f] ___, I will probably go by [name_u]Abby[/name_u] at a work place. However, a couple of years ago, I did think that [name_u]Abby[/name_u] was very childish, and I was planning to go by [name_f]Abigael[/name_f] as an adult. My longer name gave me the option to change my mind about that.
My younger sister, though, is named [name_f]Leah[/name_f] and doesn’t have any nicknames or alternative “options” for when she’s older. And it doesn’t seem to bother her at all. As for my youngest sister, her name is [name_f]Eliana[/name_f] and goes by [name_f]Eliana[/name_f] and [name_f]Ellie[/name_f] pretty much interchangeably. For her, the full name came before the nickname (my mom knew she loved [name_u]Abby[/name_u] and named me [name_f]Abigael[/name_f] specifically so she could call me that. With [name_f]Ellie[/name_f], she knew she loved [name_f]Eliana[/name_f] and decided [name_f]Ellie[/name_f] would make a cute nickname) but it was the same idea in that [name_f]Eliana[/name_f] gives her the option of a longer, fuller name.
The whole “options for the future” thing isn’t something I really think about when finding my favorite names though. My top girls, [name_f]Iris[/name_f] and [name_f]Maeve[/name_f], don’t have longer or shorter versions that could be more than pet names. [name_u]August[/name_u], I suppose, has Auggie or [name_u]Gus[/name_u] but I dislike both of those, so I’d probably just call him [name_u]August[/name_u]. But I don’t like [name_m]Augustus[/name_m] as much, so although that might give him more options, I’d opt for [name_u]August[/name_u] there too. I like many nickname names such as [name_f]Sadie[/name_f] and [name_u]Nico[/name_u] but dislike [name_f]Sarah[/name_f] and [name_m]Nicholas[/name_m], so I don’t care if the latter two would give them more “options”, I would go with the ones I actually enjoy.
I definitely think some people go over the top when suggesting longer names for “options”. There is nothing that says [name_f]Cora[/name_f] has to be short for [name_f]Coraline[/name_f] or [name_f]Cordelia[/name_f]. Sometimes, I get the impression that the poster just prefers the name [name_f]Cordelia[/name_f] over [name_f]Cora[/name_f], and thus tries to justify why the person should use it.
As for your other question, I don’t think the line is clear. I guess nicknames are names that are seen as typically belonging to a formal name. For example: [name_u]Abby[/name_u] to [name_f]Abigail[/name_f], [name_f]Liz[/name_f] to [name_f]Elizabeth[/name_f], [name_m]Nate[/name_m] to [name_m]Nathaniel[/name_m]. The names could stand on their own but people usually associate them as being shortened versions. I think it’s up to the individual to determine what names are nicknames and what names stand by themselves. I know more boys named [name_m]Liam[/name_m] than I do boys name [name_m]William[/name_m] who go by [name_m]Liam[/name_m]. I also know more boys called [name_u]Drew[/name_u] than [name_m]Andrew[/name_m] nn [name_u]Drew[/name_u]. However, I find [name_f]Ellie[/name_f] to be more of a nickname and would assume any [name_f]Ellie[/name_f] I meet is short for [name_f]Eleanor[/name_f] or [name_f]Elizabeth[/name_f] or [name_f]Eliana[/name_f], despite the fact that [name_f]Ellie[/name_f] is a common full given name. I think that it is in the eye of the beholder.