I was taught that Siddham is used in modern cultural context of Japan as a system of writing the Buddhis texts and (or) mantras.

What is the exact context of its usage? Is it applicable at all?

My apologies for bothering you with such a question, but I do lack knowledge of the topic.

  • Why the close votes? This question is not off topic! – Ataraxia Oct 5 '13 at 14:21
  • 2
    Why is it on-topic? – Flaw Oct 12 '13 at 3:27
  • This question doesn't appear to be about the Japanese language. If you feel that I'm mistaken, please vote to reopen and leave a comment explaining why you think this question is on-topic. Thank you! – snailcar Dec 30 '13 at 4:46
  • Siddham is used by some Japanese Buddhist sects to write Sanskrit. It is not used to write Japanese. So the question is off-topic. – user763305 Jun 8 '14 at 11:26

According to Wikipedia's article on the Siddhaṃ alphabet:

In Japan the writing of mantras and copying of Sutras using 
the Siddhaṃ script is still practiced in the esoteric Buddhist 
schools of Shingon and Tendai as well as in the syncretic sect of Shugendō.

... and

A recent innovation is the writing of Japanese language slogans 
on T-shirts using Bonji. Japanese Siddhaṃ has evolved from the 
original script used to write sūtras and is now somewhat different 
from the ancient script.

While your question focuses on modern use of Siddham in Japan, for a historic reference of it's introduction to China and Japan see Siddham in China and Japan - Sino-Platonic Papers. One of the things this paper shows is how Japanese is heavily derived from not just Chinese but from Siddham as well (somewhere around page 102).

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