needed在句子中既詞性

Needed 是 past participle。 Past participle 是可以作形答詞用。例如: broken window, unidentified person, etc. 此外， present participle 亦可作形答詞用，例如: sleeping dog, falling star。
As needed 中 needed 是形容詞。
As need、as approve同as plan 都錯因為Need 是動詞及名詞；approve 是動詞；paln 是動詞及名詞，而在這裡 as 後面是應該用形容詞。

係 adj~
因為as needed 係指~ as (the instructions are already/just) needed
唔想句句子太長, 所以咁寫啦~

而 as approved, planned 都一樣啦~ 因為都係想講之前d野係經過 approved, planned~
as need/ approve/ plan 當然係錯啦~
因為例如instructions 咁, 佢need 係要用passive voice先arm ga 嘛~ 所以, 要用 needed 啦~

When verbs are added "ed"s, in most cases, they're turned into their perfect tense. In this way, they become adjectives. You know the passive voice? They're exactly the same.

As needed is the shorter version of "If it is needed from you". Basically, it's telling you to put the specific instructions IF YOU NEED TO DO SO.

As I mentioned, needed, approved, and planned are all the adjective versions of their infinitives to need, to approve, and to plan. To say as need/approve/plan would mean something completely different, as those are two different types of words (one bing adjective and the other verb) they serve different functions. Grammatically it's correct if SOMETHING is placed in between AS and the verb (as SOMETHING/SOMEONE need/approve/plan), but it'll mean something else than as needed/approved/planned.

A good question though - it's quite difficult to explain the difference, especially when such thing doesn't exist in the Chinese language.