I hope some automatic translator can help with this:
"Výslovnost tvarů slovesa býti: jsem, jsi, jsme, jste, jsou. Trávníček žádá, aby se počáteční j- nevynechávalo, i když je tato výslovnost poněkud obtížná (s. 48). Z jeho formulace však není dosti jasné, zdali tuto výslovnost žádá i v případech, kdy předchází na př. slovo zakončené souhláskou (byl jsem). Jedlička konstatuje, že v přítomných tvarech slovesa býti se někdy j- nevyslovuje ani ve spisovné češtině. Zpravidla se však vyslovuje j- v těchto tvarech po samohlásce (byla jsem). VSČ ještě zpřesňuje pravidlo o výslovnosti těchto tvarů: počáteční j- se vyslovuje tehdy, jsou-li tyto tvary přízvučné (Jste to vy? Jsem!), dále ve spojení se záporkou ne- (nejsou), anebo následují-li po slově zakončeném samohláskou (kromě i). Všude jinde dáváme z fonetických důvodů (obtížná výslovnost) přednost výslovnosti bez j-." (1956)
Based on what we've learned so far, whether it is KateřinA or KateřinO depends on whether she is the subject of the sentence (Kateřina) or you're addressing her (Kateřino). In this exercise, it should be KateřinO.
Names decline like other nouns, so she could be lots of other things, too! :-)
"Kateřino, kde jste?" can also refer to one person. If Kateřina is someone with whom the speaker does not have a close relationship, or if there is a "status" difference between Kateřina and the speaker, jste could be used instead of jsi. It is the more formal way of addressing her, in a case, for example, when she is your manager at work.
The "status" difference is a little bit of a misconception - it implies that the "lower status" person would use VY towards the "higher status" person, and the "higher status" person would use TY towards the "lower status" person - an asymmetrical/hierarchical use. This is roughly how it was done over 100 years ago, but no longer, since the Czech society is very egalitarian.
Tykání (ty jsi) and vykání (vy jste) are both used mutually among adults of any status. When you use VY with your boss/manager, he/she will also use VY back with you. We use VY mutually with any stranger we meet on the street, with most neighbors, and basically anyone who is not a friend. When two people decide they want a more informal/casual/personal/close relationship, they will start using TY - again mutually.
TY/VY is only used asymmetrically between kids and adults, i.e. children use VY when talking to most adults. They use TY towards family members and also some friends of the family who explicitely give the children permission to address them as TY (and some kids may be uncomfortable with that). On the other hand, everybody uses TY when talking to children. The closer a person gets to adulthood, the more people will gradually switch to VY when addressing them. Some older people might stick to TY even when talking to people in their early twenties.
Summary: the distinction is about age and familiarity/formality, not status per se.
It can be both. And since the first name is used here ("Kateřino"), it's actually more likely that the question refers to more people (where are you guys). The combination of the polite/formal VY (French vous) with first name address is possible, but not very common in today's Czech.
By the way, DPan76, I often get a mail with your comment but when I go to the forum to answer it, it's no longer there. Earlier today it happened with the "Yesterday she was looking for her body." question.
But if your name is Marko, the vocative will be the same Marko even in Czech :D (because Czech has no masculine nouns ending in -o).
On the other hand, if you were Czech, your name would be Marek and that would then change to Marku in the vocative.
And feminine nouns ending in -e in Czech also have the same form in the vocative, e.g. Lucie, Marie, Žofie (nom.=voc.).