It's $100 for your onboard account....so everything you buy can go towards it.....shore excursions, drinks, things you buy at the store, and of course tips! So it will go fast especially tips being $10 a day per person and shore excursions can be $100
With a shipboard credit, you start your cruise with a credit of $100 and everything you charge, EVERYTHING, including tips, goes on your account. Yes, the $100 is like a nice bonus, for sure. But like the other posters state, it's essentially gone before you know it with the $10/day automatic gratuity.
Keep in mind, however, that a passenger has the option of having that automatic gratuity removed, and tipping as they wish, where they wish. But we feel that the $10/day is AOK with us, and if a crew member (dining room, cabin steward, for example) goes above and beyond, we recognize them additionally. But we know of passengers who prefer NOT to have that automatic gratuity and request that it be removed.
One way to look at it is that the $100 shipboard credit pays for the tips, period. :-)
So....charge what you want, it goes on your statement. Just remember to leave your credit card at the beginning or during the cruise to avoid the long line to settle your account at the end. The Purser's Office ALWAYS has to page for passengers to come "pay up" while everyone is eager to disembark that final morning. Very frustrating to have to wait for those stray passengers! Soooo much easier to leave your credit card, like you would do at a hotel.
If your credit credit is 100 each you will at least have 30 each to spend but if it's 100 total for both of you remember you will still over 100 in tips after the 100 credit is used on tips. Tips 2 people=$140 for 7 days
Sometimes TA's give an onboard credit as a thank-you for choosing them, and they can specify what it's for, like for a bottle of wine, or gift shop, or whatever. So perhaps that's what your TA meant. However, most TA's and/or others just give a shipboard credit on your account, to do with as you choose.