Bonan posttagmezon!

I have a question. I was studying Ana Pana Part 3 in Chapter 4 of the Libera Esperanto-Libro when I came across this sentence in the dialog:

Mi ne scias. Mi ne havas iun planon.

Instead of "iun", shouldn't it be "ion"? I thought "iu" meant somebody or someone, and "io" is some or something.

A similar situation occurs later in the dialog:

Mark - Mi loĝas en SoHo.
Ana - Bone, mi konas tiun urboparton.

Shouldn't it be "tion urboparton"? Can someone clarify this grammar point for me? Dankon!

In both cases, the noun is being identified. Use of -io is for unidentified (unstated) nouns.

Mi konas tiun urboparton.
Mi konas tion.
Tio estas bela.
Tiu urboparto estas bela.

Excellent. I understand now. Thank you very much, RiotNrrd!