"out of core" would need an explanation, here.

Historically, computer (which ran UNIX operating system) used "core memory".
https://en.wikipedia.org/wiki/Magnetic-core_memory

In the context, "out of core" means specifically "out of memory", actually.

Same. See above.

Same. See above.

In OpenPGP, there are two kinds of keys: primary key and subkey.

A subkey belongs to a primary key.

Primary key is the main key which has a capability of certification (optionally can have capabilities of signing, encryption, and authentication).

Subkey can have capabilities of signing, encryption, and authentication.

Having separate subkey(s) would make administration of key easier, because the use cases of signing/encryption/authentication key and primary key might be different (the former is daily use, the latter is occasionally).

With use of subkeys, a user can create/revoke new subkeys keeping the trust of primary key.

Yes, the number means the columns needed to display the term.

When the term uses two Chinese characters == four columns, 8 is correct.

See my comment for "out of core".

I mean, I think that it's somehow irrelevant nowadays to translate "core" directly in this context, because we never use core memory anymore in modern hardware.

Even the time when I started to use UNIX on Sony's workstation, it was semiconductor memory (in 1988 or so).

I'm afraid that the term "core" these days may be used in different purpose, like CPU cores.

Well, the technology itself is still available, though: https://www.core64.io/

Same, see above.

I'm not sure if 15 is correct or not.

It should be column length for: 无效的主题名称‘

It is correct when ‘ occupies one column, not when two columns.

Same problem for "core".

I see. It's just that word-for-word translation is a bit strange. The translation itself is right

Chinese quotation marks display different with different font. Sometimes one column, sometimes two columns.
So I will use english quotation marks to solve this problem in next update.