I see both "aquel" and "aquél" used in similar context and was wondering if there is any difference in meaning of those two words.
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Aquel is an adjective whereas Aquél is a pronoun.
Until 2010, the official rule was to use accent (tilde diacrítica in Spanish) to differentiate the pronoun from the adjective, as @snumpy has already said.
The (IMHO) sad news is that since 2010 the rules have been changed by RAE, and now it is recommended to avoid the use of the accent in this case, even when there is an obvious case of ambiguity: