教科書　”The Algorithmic Beauty of Plants”　の 1.1、 1.2　その他の項で散見される OL, IL, DOL は L-systemの種類・分類を意味しているが、おそらく”O” アルファベットのオー　は　数字の 0 ゼロ、”I” は数字の1　のタイプミスだと思われる。少々専門的ですが、下記のように定義されているようです。

0L – an L system with context-free rules

1L – an L system with context-sensitive rules

D0L – a 0L system which is Deterministic, that is, for each letter ‘a’, there is a single rule of the form { a -> … }

P0L – a 0L system where there is no ‘erasing rule’, that is, there is no rule where { a -> nothing }. you can also have a PD0L system

E0L – a 0L system where the alphabet is divided into terminal and non-terminal letters. all rules are of the form: { A -> a } or { A -> A }, etc. where on the left side you can only have non-terminals

T0L – a 0L system, where you have multiple sets of rules (or ‘tables’ – hence T0L). in each step, you select which ‘table’ (set) of rules you’re using. of course there can be DT0L, ET0L, etc. languages