If desired.
Avoid Denim.

Collared.

Generally the safest option.

Or similar collared top.

Avoid slogans and logos.

Avoid slogans and logos.

T-shirts should probably be avoided for most occasions as they do not have a collar which is normally one of the defining features of 'smart' dress, but can look smart, for example if worn with a jacket.

With or without matching jacket.
Avoid wearing two different patterns for the trousers and jacket - plain goes with anything.

Or similar.

Not suitable for all occasions. If in doubt avoid.

Always wear very smart or fashionable ones, if at all, and always in good condition.

Can't go wrong with black.

Normally black or brown.

If weather requires.

If desired.

A wedding or engagement ring is always allowed.

If normally worn.

If normally worn.

Wear something smart, comfortable, and inoffensive.

Spiffy Casual is derived from the term Spiffy, meaning cool, neat, unique, or interesting and has it's (sometimes ironic) origins in the archaic English exclamation 'Spiffing!'.

This suggests that one should dress up but keep it modern and trendy. A designer suit with a black shirt with or without tie or even something more creative such as mixing a T-Shirt with the suit, or wearing a very fashionable or designer jacket are all good. A lady might also want to consider a cocktail dress.

As always consider the event your are going to and who you are going to be there with.