IMHO

It's the "tersedia" part that's confusing. Should it actually be "tersedia untuk dijual" (available for sale) ?

☕ The first part, laba (rugi) yang belum terealisasi, is quite straightforward: unrealized gain (loss). As for the second part, investasi efek yang tersedia, we should open the dictionary of Kamus Besar Bahasa Indonesia (http://kbbi.web.id/sedia-2) to gain insight on the adjective:

↪ tersedia v (sudah) disediakan; sudah ada; disediakan untuk.

✔ The first and third meanings refer to availability and expendability, but the second meaning sudah ada refers to existence. Considering the context, a proper translation will then be existing investments in securities.

CYBER DIGGING ✸✸✸✸✸
https://depts.gpc.edu/Foundation/Audit&Statements/Audit 2005...
Investment income as shown on the Statement of Activities generally includes interest, dividends, capital gains (losses) and net unrealized gains (losses) on existing investments.

https://www.bundesbank.de/Redaktion/EN/Downloads/Publication...
In fact, we find that banks with higher unrealized gains on existing investments lend less than other banks.

http://bpkonstruksi.pu.go.id/berita.php?halaman=detail&kode=...
Informasi jumlah pasokan alat berat yang tersedia (existing, produk, penjualan, impor) masih berdasarkan data estimasi.

Generally used accounting terms. The word "investments" is redundant, it is understood from the English terms "securities held". Tersedia is indeed ambiguous, in accounting it is likely to refer to something in the possession of the entity.

Laba (rugi) yang belum terealisasi atas investasi efek yang tersedia - bersih

net unrealized profit (loss) of available securities investment.