There is no difference. Both sf and sfz are the abbreviations of Sforzando, which means sudden weakness and are strength terms.

Related introduction:

Venture marks usually use Italian musical terms. Composers mark detailed dynamics on the score, which can usually be divided into more than a dozen levels from the weakest to the strongest, and the intensity of each level is a relative value. Forte means strong, abbreviated as f. The more f's, the stronger. If there are five f's marked on the score, it means it is quite strong.

Extended information

Related background:

The changes in intensity in music are very detailed and complex, and it is tedious to mark all the subtle changes in intensity. It is impossible and unnecessary. Each performer will make specific and detailed changes in intensity according to his own feelings under the prompts of the composer.

Intensity changes are an important means of musical expression. It can express rich emotions and create contrast and development in music. Generally speaking, the stronger the intensity, the more tense and majestic the music is; the weaker the intensity, the gentler and more euphemistic the music.

Related terms:

1.Mezzo Piano/mp

2. Crescendo: Crescendo/cresc

3. Medium strong: Mezzo Forte/mf

4. Strong and then weak: Forte Piano/fp

Baidu Encyclopedia - Music Terms